The internet term for unsolicited electronic messages, especially advertisements, is spam.
Spam usually means unwanted messages sent to many recipients, most famously unsolicited email. Modern spam can also appear in text messages, social-media comments, forums, blogs, search results, and messaging apps. It may promote a product, spread propaganda, or attempt phishing and malware delivery; not every spam message is commercial.
The term comes from a 1970 Monty Python’s Flying Circus sketch in which Vikings repeatedly sing “Spam” while a diner menu keeps listing the canned meat. The repetition became a metaphor for an unwanted message overwhelming normal conversation. The first widely recognized mass email spam was sent by Gary Thuerk on May 3, 1978, advertising Digital Equipment Corporation computers to 393 ARPANET users.
“Spam” is the unwanted message itself; “spamming” is the act of sending it. A newsletter someone deliberately subscribed to is not normally spam, even if it later becomes annoying.