First, lets define it literally and for that we need the characters/ideograms that are used to denote “Goju Ryu Karate [呉寿竜].” The variety of characters start with, , and mean “Fifty [五十].” The characters/ideograms for Goju are, “呉寿,” the first character means, “Give; do something for,” the second character means, “longevity; congratulations; one’ natural life.” Then there are the characters/ideograms used by the practitioners of Goju-ryu karate, i.e., “剛柔流,” the first character means, “Sturdy; strength,” the second character means, “Tender; weakness; gentleness; softness,” the third character means, “current; a sink; flow; forfeit.” All three characters/ideograms are given to mean, “Goju-ryu (karate school).”
Second, lets go with the characters “剛柔,” which are defined alone as, “hardness and softness.” The characters alone still mean the same as the last definition given above, i.e., the first character means, “Sturdy; strength,” the second character means, “tender; weakness; gentleness; softness,. It is well-known and accepted by all who know of this style or system of Okinawan karate to be a “Hard-Soft School” of karate. It is also obvious that those who practice goju tend to be considered “Hard Style Karate.” Is this accurate is the question needed to truly understand, “What is Goju?”
I can only answer that from my personal perceptions, perspectives and understandings from my personal studies of karate and martial arts. I suspect my answer will not actually jive with the definition given by Goju Ryu practitioners, etc. but remember, it is just my view, opinion and personal understanding.
Goju as hard-n-soft is about the fundamental principles of martial arts with emphasis on the principle of philosophy as it explains yin-yang. When we say hard-and-soft or hard-n-soft we are a bit skewed as to its meaning. It should be as it is presented in the characters/ideograms as, “hard soft or hard-soft” with the hyphen in between meant to connect them similar to the yin-yang philosophy.
Hard-soft/Yin-yang, yin being the soft side while yang being the hard side. When you take that philosophy you expand the meaning and its symbolism, etc. beyond merely the hard as in hard as a rock or soft as in the softness of cotton balls.
Lets take a look at the physiokinetic principle for a moment to see how that relates to the term, characters and ideograms of goju or hard-soft. Take a look at the list of sub-principles of physiokinetics:
Physiokinetic sub-principles are, “Breathing, posture, triangle guard, centerline, primary gate, spinal alignment, axis, minor axis, structure, heaviness, relaxation, wave energy, convergence, centeredness, triangulation point, the dynamic sphere, body-mind, void, centripetal force, centrifugal force, sequential locking and sequential relaxation, peripheral vision, tactile sensitivity, rooting, attack hubs, attack posture, possibly the chemical cocktail, Multiple Methodologies [actual tactics and attack methodologies of impacts, drives (pushes), pulls, twists, takedowns/throws and compression, etc. are best for stopping a threat]” Please note I have set in bold those sub-principles I would relate directly to my meaning of goju.
Note II: I underlined the sub-principle that I have observed through the Sanchin kata where a outcome of its practice is locking and relaxation, i.e., in other words hardening the body with dynamic tension then relaxing the body through a positive relaxation of the muscles, tendons, cartilage, etc.
Note III: You may begin to perceive something in this answer that relates to an Okinawan term and trait considered the essence of hard and soft application, i.e., Chinkuchi.
All the bold and underlined sub-principles are obvious in Goju, especially when observing the Sanchin Kata. This does not mean the other sub-principles are not a part of this but rather coupled in a harmonious way as a holistic whole or wholehearted application of principles that make hard-soft and all other aspects of martial arts and karate disciplines work. You can call it Chinkuchi; you can call it body mechanics; you can call it dynamic tension or isometric but they all are necessary to make any physical aspect of the disciplines work then you have to add in the other principles of, “Theory, Technique, Self-defense, and Chemical Cocktail.”
Yin-yang is a duality that complements each other while remaining a singularity, the nature of yin-yang as is the nature of hard-soft. As Goju demonstrates the extremes of hard-soft as with yin-yang can achieve a 99% state of either hardness or softness as appropriate but as they are practiced and applied there is always a smidgeon of hard and soft present in those extremes as symbolized in yin-yang by the white/black “Dots or holes or circles” within the larger white/black spaces that represent the extremes of yin-yang therefore hard-soft.
As one practices, trains and applies Goju, like hard-soft or yin-yang, there is a state that is constantly moving from the balance point where one side and the other cross over and blend into a balance of yin-yang or hard-soft, then continue that constant motion toward the other extreme of soft-hard or yang-yin.
Goju is an expression created from one karate luminary of an older Okinawan culture and discipline along with their unique belief system that gave birth to the practice, training and application of Goju-Ryu Karate-Jutsu-Do. Think of this when next you observe a Goju demonstration, i.e., the external hard presentation is the Yang exposed while the soft side of Goju, the yin, is harder to discern except with experience that resides on the inside. The transitions of the external can be observed while still seeming hard, but not. Look close, you will see the dynamic complementation this practice provides.
I suspect that is why some degree of the Goju of karate exists in all of the Okinawan karate disciplines, it is a matter of degree that is in and of itself also a rendition of Goju, hard-soft and yin-yang.
Here are the principles, take some time to scrutinize them and I will bet you they will connect to Goju and Yin-Yang, etc.
PRINCIPLE ONE: PRINCIPLES OF THEORY (Universality, Control, Efficiency, Lengthen Our Line, Percentage Principle, Std of Infinite Measure, Power Paradox, Ratio, Simplicity, Natural Action, Michelangelo Principle, Reciprocity, Opponents as Illusions, Reflexive Action, Training Truth, Imperception and Deception.)
PRINCIPLE TWO: PHYSIOKINETIC PRINCIPLES (Breathing, posture, triangle guard, centerline, primary gate, spinal alignment, axis, minor axis, structure, heaviness, relaxation, wave energy, convergence, centeredness, triangulation point, the dynamic sphere, body-mind, void, centripetal force, centrifugal force, sequential locking and sequential relaxation, peripheral vision, tactile sensitivity, rooting, attack hubs, attack posture, possibly the chemical cocktail, Multiple Methodologies [actual tactics and attack methodologies of impacts, drives (pushes), pulls, twists, takedowns/throws and compression, etc. are best for stopping a threat]???see below)
PRINCIPLE THREE: PRINCIPLES OF TECHNIQUE (techniques vs. technique, equal rights, compliment, economical motion, active movement, positioning, angling, leading control, complex force, indirect pressure, live energy and dead energy, torsion and pinning, speed, timing, rhythm, balance, reactive control, natural and unnatural motion, weak link, non-telegraphing, extension and penetration, Uke. Multiple Methodologies [actual tactics and attack methodologies of impacts, drives (pushes), pulls, twists, takedowns/throws and compression, etc. are best for stopping a threat])
PRINCIPLE FOUR: PRINCIPLES OF PHILOSOPHY (Mind [mind-set, mind-state, etc.], mushin, kime, non-intention, yin-yang, oneness, zanshin and being, non-action, character, the empty cup.)
PRINCIPLE FIVE: PRINCIPLES OF SELF-DEFENSE (“Conflict communications; Emotional Intelligence; Lines/square/circle of SD, Three brains (human, monkey, lizard), JAM/AOJ and five stages, Adrenal stress (stress induced reality based), Violence (Social and Asocial), Pre-Attack indicators, Weapons, Predator process and predator resource, Force levels, Repercussions (medical, legal, civil, personal), Go-NoGo, Win-Loss Ratio, etc. (still working on the core sub-principles for this one)”Attitude, Socio-emotional, Diplomacy, Speed [get-er done fast], Redirected aggression, Dual Time Clocks, Awareness, Initiative, Permission, multiple attack/defense methodologies (i.e., actual tactics and attack methodologies of impacts, drives (pushes), pulls, twists, takedowns/throws and compression, etc. are best for stopping a threat)
PRINCIPLE SIX: CHEMICAL COCKTAIL: (Attacked Mind, Train It, Breath It Away, Visualize It Away, Sparring vs. Fighting, Degradation of Technique/skills, Peripheral Vision Loss, Tunnel Vision, Depth Perception Loss/Altered, Auditory Exclusion, Weakened legs/arms, Loss of Extremity Feeling, Loss of Fine Motor Skills, Distorted Memory/perceptions, Tachypsychia (time slows), Freeze, Perception of Slow Motion, Irrelevant Thought Intrusion, Behavioral Looping, Pain Blocked, Male vs. Female Adrenaline Curve, Victim vs. Predator, The Professional, Levels of Hormonal Stimulation, ???)