性 xìng is a pictophonetic compound: the heart radical 忄 carries the meaning, while 生 (shēng) supplies the sound. The 忄 on the left is the upright form of 心 (heart, mind), and it marks the character as something that lives in the inner self. The phonetic 生, meaning to be born, to grow, lends its pronunciation; yet here the two halves rhyme in sense as well, for 性 is precisely what one is born with in the heart. The simplified and traditional forms are identical, eight strokes unchanged across the script.

The core idea is innate nature: the disposition a thing carries from birth, what it cannot help being. From that single root the senses spread along four lines:

1. Nature, character, disposition. The temperament of a person: 性格 xìnggé (character, personality), 个性 gèxìng (individuality), 本性 běnxìng (innate nature), 性情 xìngqíng (temperament).
2. Sex, gender. The born category of male or female: 性别 xìngbié (sex, gender), 男性 nánxìng (male), 女性 nǚxìng (female), 异性 yìxìng (the opposite sex).
3. Property, quality. The inherent character of a thing: 性能 xìngnéng (performance, properties), 弹性 tánxìng (elasticity), 毒性 dúxìng (toxicity).
4. The abstract -ness / -ity suffix. Attached to a word, 性 turns it into the quality of being so: 可能性 kěnéngxìng (possibility), 重要性 zhòngyàoxìng (importance), 安全性 ānquánxìng (safety, safeness).

The thread is what lies inborn. A person's temperament is the nature of their heart; the male or female category is the nature of the body; the elasticity or toxicity of a substance is the nature of the material; and as a suffix, 性 simply names the abstract nature, the -ness, of whatever it follows.

The oldest and richest face of 性 is the inner disposition of a person. 性格 xìnggé is the everyday word for character or personality, the settled set of traits by which we know someone. 个性 gèxìng narrows to what makes a person distinct from everyone else, their individuality. 本性 běnxìng reaches deeper still, to the nature one is born with and rarely sheds. 性情 xìngqíng colors the whole with feeling, the temperament as it shows in mood and reaction. Together they map the heart from its deepest root to its outward grain.

Because sex is a category one is born into, 性 also names it. 性别 xìngbié is the neutral term for sex or gender, the box on a form, the category itself. From it the language builds the everyday pair 男性 nánxìng (male) and 女性 nǚxìng (female), more formal than 男人 and 女人 and common in writing about people in the aggregate. 异性 yìxìng names the opposite sex, the counterpart category. Here 性 has narrowed from the whole inner nature to one fixed dimension of it, the dimension of male and female.

Extend "nature" from people to things and 性 names the inherent property of a material or system: 性能 xìngnéng (the performance or capabilities of a machine), 弹性 tánxìng (elasticity, the quality of springing back), 毒性 dúxìng (toxicity). Push the same logic one step further and 性 becomes a pure suffix, the workhorse that turns an adjective or verb into an abstract noun, the equivalent of English -ness and -ity: 可能性 kěnéngxìng (possibility), 重要性 zhòngyàoxìng (importance), 安全性 ānquánxìng (safety). This is one of the most productive word-building tools in modern Chinese, the standard way to name a quality in the abstract.

Hold one picture at the center of 性: a heart (忄) that was born (生) a certain way. That is innate nature, what a thing carries from birth and cannot easily shed, and it is the seed of every sense.

Turn the heart toward a person and you get character: 性格 personality, 个性 individuality, 本性 the deep true self, 性情 temperament. Narrow that born nature to one fixed dimension, male or female, and you get sex and gender: 性别, 男性, 女性, 异性. Carry "nature" over to things and you get inherent property: 性能 performance, 弹性 elasticity, 毒性 toxicity. Finally, strip the word down to a pure tool and 性 becomes the -ness suffix that names any quality in the abstract: 可能性 possibility, 重要性 importance, 安全性 safety. Whenever you meet 性, ask whose nature this is, the heart's, the body's, the material's, or simply the abstract nature of some quality.