The RMB etiquette dispatch says that "Spamming is probably best described as a post that is either meaningless and unnecessary, a.k.a. a post that repeats the same thing multiple times, multiple posts of the same thing, a series of posts that have no apparent meaning,etc."
Google defines spam, when used as a verb, as "send the same message indiscriminately to (large numbers of recipients) on the Internet."
The NationStates One Stop Rules Shop says "There are many types of spam, but the general definition is making a post that doesn't have any sort of use. Excessive bumping, tagging, and random posts with irrelevant information are all examples of spam. The same applies to in-game posts on regional message boards and telegrams." (It is without a doubt excessively giving me notifications.)
Urban Dictionary defines spam as "spam spam spam spam spam spam spam spam spam spam spam spam spam spam spam spam spam...", likely because it's repetitive.
Wikipedia says that "Spamming is the use of messaging systems to send an unsolicited message (spam), especially advertising, as well as sending messages repeatedly on the same site."
Oxford Dictionaries define spam as "Irrelevant or unsolicited messages sent over the Internet, typically to a large number of users, for the purposes of advertising, phishing, spreading malware, etc.", while also defining unsolicited as "Not asked for; given or done voluntarily." and advertisement as "A person or thing regarded as a means of recommending something." (which would work because the telegrams recommend I stop double posting)
Twitter says that "Spam can be generally described as unsolicited, repeated actions that negatively impact other people."