1.
time
The general concept of time, or a period/amount of time. 時間がない (no time), 時間がかかる (takes time).
2.
hour(s)
Used as a counter for hours. 三時間 (three hours).
時間 has two primary uses: as a general noun meaning "time" and as a counter for hours.
SENSE 1 - TIME (GENERAL):
- 時間がある/ない: to have/not have time
- 時間がかかる: to take time
- 時間を無駄にする: to waste time
SENSE 2 - COUNTER (HOURS):
- 一時間: one hour
- 二時間: two hours
- 何時間: how many hours?
KEY DISTINCTION:
- 三時: 3 o'clock (clock time)
- 三時間: 3 hours (duration)
SPECIAL USAGE:
時間 can mean "class period" in school: 一時間目 (first period)
COMPARE:
- 時間: time (general), hours (duration)
- 時: time, moment, when