A lettermark is a logo built from a business's initials rather than its full name — typically two or three letters, styled distinctively as a standalone mark.
Lettermarks are most effective for businesses with long or unwieldy names, or businesses that have already built strong name recognition and can afford to lean on initials alone. Without that existing recognition, a lettermark risks being memorable as a shape but meaningless as a message.
Because they're compact, lettermarks translate well to small applications like favicons and app icons, which is part of why many established companies eventually adopt one alongside their full wordmark.