時計 [とけい] – watch; clock

• 時 [ジ・とき] – time; hour
• 計 [ケイ・はかる] – measure

A device that measures time is a clock.  Makes sense, eh?  If you want to specify a watch rather than a clock, prefix the word with the word for arm:  腕時計 [うでどけい].  Notice also that the pronunciation of 時計 changes from とけい to どけい.

The character 時 is a very cool one in my opinion; it makes it easy to see how (some) kanji are constructed.  If you take a good look at it, you can see that it is composed of two parts:  日 [ニチ・ひ] – day; sun, and 寺 [ジ・てら] – temple.

All throughout the world, in various cultures, temples have been used to measure time.  Imagine watching the sun through a slit in the temple wall and using that as a means of measuring the passage of time.  (Told ya it was cool.)