When I decide to ask this to students varies depending on when I feel I'm lost on where to take them next.
For me, singing is a mode of expression. It helps express beyond what words can't express (can't remember exact quote and by whom). It's probably why I always had a wide range of expression in my voice even when starting off singing because that's what music is for me.
Today, I asked a student what singing is for him. It was simply "making beautiful sounds". He ALWAYS does this! Something I had slight jealously toward. If we compare his singing and mine when I was his age: his singing more beautiful, my singing more expressive. I guess it goes back to one of my earlier questions on technique or expression first (expression itself is also a technique in my opinion). He's likely to get lead roles earlier on than I was. I had character roles because I didn't have a "lead voice". And my teacher told me to start getting out of those roles when I started developing the voice for more serious roles. I don't know though. I like being silly on stage.
How you define why you sing, how you sing, who you sing for, what you sing for, etc will influence your performance. LOL the "Who, what, where, when, how, and why" of singing!
I'd like to know how you guys define it for yourself!