Retro illustration style is a broad term that refers to a style of drawing and design that emulates the aesthetics of a past era, most commonly the mid-20th century (from the 1920s to the 1970s). It's a style that evokes a sense of nostalgia, often for a simpler, more optimistic time.

This style isn't a single, uniform look but rather a collection of sub-styles, each with its own distinct characteristics based on the period they're imitating.