Flirting is a pretty nebulous term.

My personal definition of flirting is any positive expression or behavior which:

1. Is an exception to your typical behavior or affect.
2. Targeted at a specific person, typically someone new.
3. Is heightened or marked by increased volume, nervousness, etc.

examples:

A typically reserved guy pulling you aside and animatedly asking about your interest.

A woman who normally doesn’t touch you is repeatedly teasing you about your shirt, pulling on the fabric.

A good friend begins to repeatedly and unexpectedly invite you over for one on one movie nights despite obvious inconveniences.

That’s just my opinion, though. I believe most people are looking for these heightened expressions of flirting to confirm interest.