In the context of website development or application development a wireframe black and white, deliberately simplified line drawing of the layout of a website or application.

This is sometimes mistakenly synonymous with website prototyping.

Example

Website Wireframe Example by The Portland Company

How are wireframes used?

Wireframing is one step in a professional website design process. It's a time and cost efficient method of scoping a project opposed to making layout, behavior flow and feature level changes later in the development process.