片刻 (piàn kè) is a noun meaning 'a moment' — a very short period of time, often used in written or formal speech. It is similar to 一会儿 (yīhuìr) but 片刻 is more literary and always a noun, while 一会儿 is colloquial and can also mean 'soon'. 一时 (yīshí) overlaps but often means 'temporarily' or 'for a while' in set phrases.