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.)