In terms of architectural design, a concept is the main theme, the main idea that guides and leads the creation of a projects form or function or all of them. A concept is what gives character to the project at hand.
Following examples stay strong to the concept.
Visual concept: Visual concepts stand out easily. They don’t need to be explained.
Functional concept: Functional concepts might not be seen, but they add spatially to a building and improve upon the overall comfort level. Functional concepts are more experiential than visual.