Well, it’s not exactly an objective thing, but the most concise way I’ve heard it described is “plausible deniability”. It’s always just a joke until… it isn’t.

Flirting is mainly about showing interest but also always giving the other person a graceful out. It’s asking out without asking out, so nobody has to actively reject the other, but instead can just stop responding.

Also, flirting is fun! Just have fun with it.