An alteration is a change made to something, usually to improve it or make it work better for a new purpose. When a tailor makes alterations to a dress, they might shorten the hem or take in the waist so it fits perfectly. When an architect plans alterations to a building, they're modifying the structure, perhaps adding a room or changing the layout.

The word suggests deliberate changes rather than accidental ones. You wouldn't say time and weather made alterations to an old fence (that's just decay), but a carpenter who repairs and redesigns that fence is making alterations. Alterations can be small, like adjusting a recipe by adding more salt, or significant, like the major alterations a historic theater undergoes when it's renovated to add modern technology while keeping its original character.

You'll often encounter this word when something needs to be adjusted to fit better or work differently than it was originally designed to do. People make alterations to documents, plans, and clothes. Scientists make alterations to experiments when they need better results. The key idea: something existed in one form, and someone intentionally changed it to serve a purpose more effectively.