A timepiece is a clock or a watch — anything made to show the time. In plain meaning it covers both, which is its whole reason for existing: it is the word you reach for when you want one word for wristwatches, pocket watches, wall clocks and grandfather clocks together. But the everyday feeling matters more than the definition here. Nobody in ordinary conversation says 'What time is it? Let me check my timepiece.' They say watch, or clock. Timepiece is formal, a little old-fashioned, and slightly grand — you meet it in museums, in auction catalogues, in the writing of collectors, and above all in advertising for expensive watches, where 'a fine Swiss timepiece' sounds more serious than 'a watch'. Because of that, using it about a cheap plastic watch is usually a joke. Note also that it is a plain noun, countable, and normally written as one word.