A daydream is what the tin says: a dream you have by daytime 
It can hardly be defined in other ways without losing in broadness of meaning, but for most, it’s letting their imagination flow while they usually detach themselves from what’s going on outside. It has varying degrees of “realness” for different people, depending on how vivid their imagination is (and yes, it’s “vivid” as if referring to dreams), and apparently it also has varying degrees of lucidness.
There’s a very interesting topic named Lucid DayDreaming, you can check that out for more info ^^