Irrelevant or unsolicited messages sent over the Internet, typically to a large number of users, for the purposes of advertising, phishing, spreading malware, etc.

https://en.oxforddictionaries.com/definition/spam

Also,

Unwanted or intrusive advertising on the Internet.

https://en.oxforddictionaries.com/definition/spam

Technopedia explains Spam as,

The most common form of spam is email spam, but the term also applies to any message sent electronically that is unsolicited and bulk. This includes: instant message spam, search engine spam, blog spam, Usenet newsgroup spam, wiki spam, classified ads spam, Internet forum spam, social media spam, junk fax spam, and so on.

https://www.techopedia.com/definition/1716/spam