A visual identity is a brand's coherent system of graphic elements – logo, colour palette, typography, iconography and the rules for using them – that makes a company recognisable at every customer touchpoint.
A good visual identity answers not only "what do we look like?" but, more importantly, "how do we stay consistent?": it defines the rules that keep a business card, website, presentation and packaging visually consistent.
In practice an identity covers: the mark and logotype with variants, colour palettes (with print and screen values), typeface selection, composition rules, photography and illustration style, and usage examples. The whole is documented in a brand book.
Why it matters: customers build trust through repetition. A brand that looks different every time looks like several brands – and none of them gets remembered.