代 dài is a pictophonetic compound: the person radical 亻 carries the meaning, while 弋 (yì) supplies the sound. The 亻 on the left marks the character as something to do with people, and the core idea is one person standing in the place of another. The phonetic 弋, an old word for a stake or a tethered arrow, lends only its pronunciation, not its meaning. The simplified and traditional forms are identical; only one glyph exists.

From the image of one figure replacing another, the senses unfold along two paths, the personal and the temporal:

1. To replace, to substitute. One thing or person put in the place of another: 代替 dàitì (to replace), 取代 qǔdài (to supersede), 代理 dàilǐ (to act as agent).
2. On behalf of. To act in another's stead: 代表 dàibiǎo (to represent), 代办 dàibàn (to handle on someone's behalf), 代购 dàigòu (to buy for another).
3. Generation. One human cohort succeeding the one before: 一代 yī dài (a generation), 后代 hòudài (descendants), 上一代 shàng yī dài (the previous generation).
4. Era, period, dynasty. A stretch of time defined by what fills it: 时代 shídài (era), 古代 gǔdài (ancient times), 现代 xiàndài (modern), 年代 niándài (decade), 朝代 cháodài (dynastic period).

The thread is succession: one standing where another stood. A person who steps into your role replaces you; a person who acts in your name speaks for you; the children who follow you are the next generation; and the long sweep of generations stacked one on another is the procession of eras.

The most concrete face of 代 is the act of putting one thing in the place of another. 代替 dàitì is the everyday verb for replacing: you stand in for an absent colleague, one material replaces another. 取代 qǔdài is sharper and more final, a supersession in which the new thing pushes the old one out altogether. 代理 dàilǐ narrows the idea to authority: to act in an official capacity for someone who cannot, an agent or a stand-in manager. All three share the single image of a substitution, differing only in how complete and how formal that substitution is.

Closely tied to substitution is the gentler idea of acting in someone's name. Here 代 does not push the other out but speaks or acts for them while they remain. 代表 dàibiǎo is the central word: to represent, and the representative who does so, from a class monitor to a member of a national congress. 代办 dàibàn is to handle a piece of business on another's behalf, the everyday errand done in someone's stead. In both, the doer is a stand-in by consent, carrying the absent party's interest forward.

Stack one substitution on the next, generation after generation, and 代 becomes a unit of time. At the human scale it counts cohorts: 一代 yī dài (a generation), 后代 hòudài (the descendants who come after), 上一代 (the elders before us). Widen the lens and the same syllable names the great stretches of history: 时代 shídài (an era or age), 古代 gǔdài (antiquity), 现代 xiàndài (the modern age), 年代 niándài (a decade), 朝代 cháodài (a dynastic period). Time, in this character, is measured by who fills it, one set of people handing the world on to the next.

Hold one picture at the center of 代: a person (亻) stepping into the place another has left. That single act of standing in for someone is the seed of every sense.

Have the newcomer take over the job and you get substitution: 代替 to replace, 取代 to supersede, 代理 to act as agent. Have them act in the other's name while the other still stands behind them and you get the gentler "on behalf of": 代表 to represent, 代办 to handle for, 代购 to buy for. Now let the substitutions repeat down a line of people, parent to child to grandchild, and you get the generation: 一代, 后代, 上一代. Stack those generations into the long march of history and you get the era: 时代, 古代, 现代, 朝代. Whenever you meet 代, ask who is standing in whose place, and over how long a stretch of time.