ESP (email service provider)
The service that stores your contact list and actually sends your campaigns.
An ESP is the platform you send from — Mailchimp, Klaviyo, Brevo, HubSpot and the rest. It holds the subscriber list, queues the send, handles unsubscribes and reports what happened.
An ESP is not the same thing as an email builder. The builder is where the email is designed; the ESP is where it is sent. Many ESPs include a basic builder, which is why the two get confused, and why templates designed elsewhere have to be exported into the ESP to go out.
A related term is MAP, a marketing automation platform: an ESP with campaign logic on top — branching, scoring, CRM integration.