A game is is a challenge that you can overcome, mostly using skill, and which gives you a reward when you do.
This is a quote I've arrived at since the 90s and through my degree in games development until this final quote from 2014. It separates, among other things, what we thought of as games in the 1980s and think of games now in 2023 from e.g. slot machines and other things.
Games that meet these criteria will be posted here, as well as some exceptions.