What is a functional limitation in a patent claim?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

A functional limitation in a patent claim is a term that recites a feature “by what it does rather than by what it is”. As stated in the MPEP, “A claim term is functional when it recites a feature ‘by what it does rather than by what it is’ (e.g., as evidenced by its specific structure or specific ingredients).”

Functional language is often used to define a particular capability or purpose served by an element, ingredient, or step in a process. It’s important to note that functional language is not inherently improper in patent claims.