The definition you chose was two-fold.
The part in bold and the part in italics.
"irrelevant or inappropriate messages sent on the Internet to a large number of recipients.
The large number of recipients is the repetition. It means the same message goes out repeatedly, just to different users, people, or addresses.
It is part of what qualifies spam as spam. It is how spam filters are able to catch spam, by scanning mail for similarity.
Yeah, I don't have time for this any longer either. It's a simple word, with a simple definition that is not at all hard to comprehend.