One of the times on the clock: five o'clock (goji). Depending on context it can mean 5 a.m. or 5 p.m.
明日の会議は５時に始まります。
Tomorrow's meeting starts at five o'clock.
毎朝５時に散歩するのが私の習慣だ。
It's my habit to take a walk every morning at five.
電車は５時ちょうどに駅を出発した。
The train departed the station at exactly five o'clock.