I thought I would provide another attempt at defining an alto using the approach of a list of things that were never said by any Alto. If you hear these things mentioned below coming from a choir they are usually coming from the soprano section. If you hear it from the alto section you can rest assured the culprit is really a mezzo-soprano.
1. That note is too low for me.
2. Could the sopranos please join us in bar 34?
3. I wish there were more unison parts to this song.
4. I need to apply my lipstick before I can sing this properly. (flip of hair)
5. I don't like using my chest voice.
6. I would rather sing this duet with a tenor than a bass.
7. The sopranos are singing too quietly.
8. Please don't ask me to help out the tenors in their beautiful tenor line on page 4.
9. What page are we on? (this is only and always heard from the Bass section)
10. Can we sing this in a different key?