定 dìng pairs the roof radical 宀 (mián) over a settling element below. The top is 宀, the pictograph of a roof with two side walls, which appears across the dwelling family (家 home, 室 room, 安 peace). Beneath it sits an element that the early forms show as something coming to rest, a thing brought down and made still within the house. The whole reads as a matter settled under a roof: brought indoors, set in place, and no longer moving. From that image of stillness fixed in a dwelling grows everything 定 means.

Three senses branch from the settled thing under the roof:

1. To decide, to set, to stipulate. Bringing an open question to rest by fixing it: 决定 juédìng (to decide), 规定 guīdìng (to stipulate), 确定 quèdìng (to confirm), 预定 yùdìng (to book in advance).
2. Fixed, settled, certain. The state of a thing that no longer wavers: 一定 yídìng (definitely), 固定 gùdìng (fixed), 肯定 kěndìng (certainly).
3. Calm, stable, composed. Inner and outer steadiness: 稳定 wěndìng (stable), 安定 āndìng (settled; peaceful), 镇定 zhèndìng (composed).

The thread running through every sense is the same: something brought to rest and held there. A decision is a question settled, certainty is a doubt settled, stability is a life or a state settled. Whether the thing fixed is a plan, a fact, or a frame of mind, 定 is the moment the wavering stops and the matter stands still.

Two of the most common certainty words in everyday Chinese both lean on 定: 一定 yídìng and 肯定 kěndìng. Both translate as "definitely," but they point in different directions. 一定 most often expresses the speaker's resolve or expectation about what will happen: 我一定去 "I will definitely go," 明天一定下雨 "it is bound to rain tomorrow." It is forward-looking, a promise or a confident forecast. 肯定 most often expresses confidence about what is already the case: 他肯定知道 "he definitely knows," 这肯定是真的 "this is certainly true." It affirms a present judgment.

The overlap is real, and in many sentences either will pass. The fine line is that 一定 carries the flavor of will and necessity ("it must be so, I will make it so"), while 肯定 carries the flavor of affirmation ("I am sure this is so"). 肯定 can also be a verb, "to affirm, to confirm," which 一定 cannot.

The third family of 定 turns the settled thing inward and outward into steadiness. 稳定 wěndìng describes a system that holds level: prices, a job, a government, a network connection. 安定 āndìng adds the warmth of 安 (peace) and describes a settled, secure way of life: 社会安定 "a settled society," 生活安定 "a stable life." 镇定 zhèndìng turns to the person, the composure that keeps a cool head in a crisis: 保持镇定 "to stay calm." All three share the core of 定, the wavering brought to rest, but they fix it at different scales: a system, a society, a self.

Picture a roof, 宀, and beneath it a thing that has finally come to rest and stopped moving. That settling under the roof is the whole character: the wavering ends, and the matter stands still.

From that one image read every sense. Settle an open question and you have decided it (决定). Settle a doubt and you are certain (一定, 肯定). Settle a price, a job, or a society and it is stable (稳定, 安定). Settle your own racing nerves and you are composed (镇定). Whenever you meet 定, ask what has just come to rest beneath the roof, and the meaning falls into place.