What is Cold email?
An unsolicited but personal email to someone you have no prior relationship with, sent one-to-one at low volume to start a conversation.
Cold email is outreach to a person who did not ask to hear from you. It is distinct from marketing email — which goes to people who opted in — in three ways that matter for deliverability: the recipient's provider has no history of them wanting your mail, the message has to read as one-to-one correspondence rather than a broadcast, and volume per mailbox must stay low enough that the sender never looks like a bulk operation.
Done well, cold email is short (under 100 words), plain text, specific to the recipient, sent from a warmed mailbox at 20–50 messages a day, followed up three or four times, and stopped the moment the person replies. Done badly it is a newsletter with the recipient's name pasted in, and it ends up in spam within days.