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]