I went to the doctor because I can't stop saying airplane jokes...
I went to the doctor because I can't stop saying airplane jokes
He said it was terminal
This is an airplane joke. It might also be a pun, but if it is, it is probably the worst pun ever. The humor is centered around the word terminal. Terminal means both "a disease that will cause death", and "the end of a transportation route" (such as the place an airplane lands). Now in this sentence, the first sense of the word makes sense, but the second sense makes no sense at all.
The pun is particularly poor and the second sentence is senseless since the second sense of the word doesn't make sense.
That being said, that doesn't really ruin the joke - it is still pretty funny because bad puns can still be funny.