一会儿

一 yī is "one," and 会 huì here is not the familiar "can / meeting" sense but an old measure of a brief span, "a moment, a spell." 一会 is literally "one spell," a single short stretch of time. The colloquial form adds 儿 ér (érhuà, the northern retroflex suffix), giving 一会儿 yíhuìr, the spoken word heard across the north and in Beijing.

Note the tone change: 一 is nominally first tone (yī), but before the fourth-tone 会 (huì) it shifts to second tone, yí, by the regular 一 sandhi rule. The same 会 of "a short while" appears in 等会儿 děnghuìr and 待会儿 dāihuìr ("in a bit"), and the bare 一会 (yíhuì, no 儿) is the written and southern form. All name the same small unit of lived time, shorter than an hour, vaguer than a minute.

一会儿 and 一下 are the classic confusion, because both can soften and both feel brief. The difference is what they measure. 一会儿 yíhuìr is a span of time, a little while, and answers "for how long": 等一会儿 ("wait a while," you picture the stretch of waiting). 一下 yíxià is a verbal-quantity word, "once / briefly," and softens the action itself: 等一下 ("wait a sec, hold on," you picture one quick beat of waiting). For an action you do briefly, 一下; for a length of time you spend, 一会儿.

等会儿 děnghuìr and 待会儿 dāihuìr are the everyday set phrases built on the same 会, meaning "in a bit, later, hold on a moment": 等会儿再说 ("let's talk later"). Think of it as a softened "later." And remember the position rule for 一会儿 itself: after the verb it is a duration ("for a while"), before the verb it is the near future ("in a while"). Same word, two readings, decided by where it sits.