Branding
Key visual
A key visual is the central visual motif of a campaign from which every other format is derived. It holds a campaign together so the billboard, the social post and the banners read as one thing.
What it means in practice
A key visual is not a logo or a tagline; it is a visual solution: composition, colour, type and how photography or video is handled. Once it exists, producing further formats is fast and cheap because nothing is invented from scratch.
When you do not need one: a one off performance ad meant to live for two weeks. There building a key visual costs more than the campaign itself, so go straight to formats.