I learnt this the hard way....everyone has very different definitions.
Mine personally is if they sleep 8+ hours in one stretch without my intervention.
That includes me going in and settling or feeding. I don't consider that sleeping through.
If they stir but self settle then I'd count that as sleeping through.
By this definition, my DS did not sleep through until he was 10 1/2 months.
A friend of mine considers it sleeping through if you only need to go in and feed them and they settle straight back to sleep! By that definition he slept through from 13 weeks....
The flaw there is that by the time he was 8 months old he was waking every couple of hours through the night. I was feeding him back to sleep each time. Trust me that is definitely NOT sleeping through! 😱