I always thought it meant what it sounds like - sleeping all night long! But apparently some sources say sleeping 5-6 hours is really "through the night" for babies. My son has been sleeping in 5-6 hour stretches (well it's usually 5 or 6 then 3) since about 9 weeks and once even slept from 10:30 to 7 am, though he hasn't done it since!

What's your definition of sleeping through the night?