Spam in a nutshell.

If you do it, it's spam.
When I do it, it's creating distance.

In practical contexts:
If an opponent misses a few times before successfully killing you, he/she is "spamming".

In linguistic contexts:
I think about the word in terms of emails. Lots of worthless emails = spam. Lots of pointless attacks = spam. Under this definition, one can technically spam any weapon, including bombs. However, to prove someone is doing so you have to prove the attacks are truly pointless and not directed. A real spammer would not change his/her attack pattern if the nearby players change their positions, just like how an email spambot does not care if the recipients actually read the mail.

In game theory contexts:
(Someone may try to argue with me about this, but w/e.)
Spam is taking advantage of game mechanics to compensate quality with quantity. Usually the quality is precision in aiming, and quantity is sword swings/bullets fired. Especially with shield cancel and ASI, it takes more time to perform an attack with 75% accuracy than it does to perform two with 50% accuracy. It's common sense to prefer the "spam" option over the precision option.
Even people who shield cancel tend to follow-up a miss with another immediate attack. There are very few people who don't spam at all because at some point in time, everyone had bad aim and got into the habit of using quantity over quality.