Bedtime is the time of the evening or night when you go to sleep. If you want a good excuse to leave a boring party, you can look at your watch and say, "Wow! It's way past my bedtime!"
The time you generally go to bed is your bedtime, although you may not have a regular bedtime, just turning in whenever you feel sleepy. Most young children have a set bedtime, which is basically the hour their parents have decided they should go to sleep. If they're lucky, they'll get a bedtime story before the lights are turned off. Bedtime dates from the 13th century, from the Old English bedd and tima, "limited space of time."