I have always defined spam as "unwanted and undesirable incoming communication, typically for financial reasons". So that includes mass emailing, but is far from restricted to it.

However, that is a definition I basically invented based on experience (i.e. How I imagine most people learn most of their vocabulary).

If it really has to be done in bulk to be spam-callable, what is the individual version of spam? Harassment?

I disagree. Whether it was done in bulk or not is irrelevant to the recipient. The easiest way to prove this is to find a message you consider to be spam. Forward it to someone of your choosing. The message previously considered to spam won't magically cease to be spam just because it's only sent to a single person.