The term 'wash' can mean several things:

Table of contents
1 Cleaning
2 Painting
3 Geography
4 Equality

Cleaning

A wash is the act of cleaning. I wash my hands in soapy water.

Painting

A wash is a light covering of watercolor on a painting. When painting three-dimensional models, they can be used to add shading.

Geography

A wash is a very small river or stream. Typically (at least in rural parts of the central United States) the term wash is used to reference a stream that only gets wet when there is rain, and dries quickly. The term does not refer to a section of land that gets wet during heavy rains; that's a flood plain.

Equality

There is a common American (also english?) phrase of, 'it's a wash'. This means the process of coming to be on equal footing with another party. If person A gives person B one dollar at the same time that person B gives person A a dollar, 'it's a wash'.