Logo
A graphic symbol or typographic mark that represents a brand, company, or project's identity.
In practice
A logo is the mark that identifies a company, product or project: a symbol, a word set in distinctive letters, or both. It must work small and large, in one color and full color, on screens and embroidered caps. A coffee shop logo that looks fine on a storefront may fail as a tiny browser tab icon, so designers test each version at the real sizes where it will appear.
Often confused with Visual Identity
A logo is a single mark; a visual identity is the whole system around it, including colors, typefaces, imagery and rules for use.
Try this
Shrink a logo you like to the size of a fingernail on screen and note which details disappear.