What does it mean for a patent application to be ‘amended’?

MPEP 203.03 defines an ‘amended’ patent application as follows:

An ‘amended’ nonprovisional application is one that having been acted on by the examiner, has in turn been acted on by the applicant in reply to the examiner’s action. The applicant’s reply may be confined to an election, a traverse of the action taken by the examiner or may include an amendment of the application.

In simpler terms, an application becomes ‘amended’ when the applicant responds to an examiner’s Office action, whether by making changes to the application, arguing against the examiner’s rejections, or choosing between different inventions (election).