A scheme is a plan or program of action, often organized and official, that can be used for a variety of purposes. It can refer to a crafty or secret plan, an underhand plot, or an intrigue. A scheme can also be a mathematical or astronomical diagram, a representation of astrological aspects of the planets at a particular time, or a graphic sketch or outline. In mathematics, a scheme is a mathematical structure that enlarges the notion of algebraic variety in several ways, such as taking account of multiplicities). Key features of a scheme include:

• Plan or program of action: A scheme is an organized plan for doing something, especially something that will bring a good result for the person or organization behind it.

• Negative connotation: Schemes often carry a negative connotation, bringing to mind devious plans and secret plots organized by scheming bad guys.

• Topological space: In mathematics, a scheme is a topological space together with commutative rings for all of its open sets, which arises from gluing together spectra (spaces of prime ideals) of commutative rings along their open subsets).

Overall, a scheme can be used for a variety of purposes, both positive and negative, and can refer to a plan or program of action, a mathematical structure, or a diagram.