In my opinion, it's not a nap if any of the following apply:

-It occurs less than an hour after you woke up
-It involves pajamas
-It occurs after 6pm
-It lasts more than 2 hours
-It is done as an escape from reality type thing rather than a need a little rest type thing
-It is a result of overindulgence in substances other than food

None of these factors apply if you are on a plane.

What is it if it is the last two? Or at a desk/in a chair?

Also, people can add nap tips. For shorter napping, longer napping, napping satisfaction/effectiveness, etc. Or is it sleeping?

I often must return to bed in the morning. My discriminator is whether I drank coffee yet or not.

If the light outside is different, it's not a nap.

I wish I could nap. I simply cannot sleep during the day. Never could.

I could sleep during the day if I ever had time to.

Rules for naps in my house are as follows:
- Clothed, if you are in pajamas, it's not a nap
- On top of the covers, using a throw blanket for warmth
- Must take place between the hours of noon and four pm, not a four hour nap, it just must take place during that time period
- Must lie (lay?) sideways along the bed, which isn't hard for me, because I am not very large and we have a king. Laying sideways conveys for me the sense that this is naptime, not sleeptime.
- All the pillows (both my husband's and my own) are piled around me to resemble a nest.

I used to take naps nearly every day for years, but rarely anymore. I sure do love me some naps, though. I surely do.

During the day, when I sleep I'm napping. When others nap they're sleeping.

It's a nap when I say it's a nap.

My nap cutoff time is more like 4.5 hours at which point it becomes "I slept on my couch accidentally"

Naps can be day or night, with or without pjs, but you have to intend to wake up soon to be a nap.
So if I go to bed at night, sleep 3 hours and wake up, that is not a nap because I meant to be sleeping the whole night.

I call anything over 60 minutes not a nap.