To alter means to change something, usually in a small or specific way. When you alter your plans, you adjust them without completely starting over. A tailor alters clothing by taking in a waist, shortening sleeves, or letting out a hem so the garment fits better.

The word suggests modification rather than transformation. Altering a recipe might mean using honey instead of sugar or adding extra cinnamon. You're changing it, but it's still recognizably the same recipe. This differs from words like transform or revolutionize, which suggest dramatic change.

You'll often hear alter in contexts where precision matters. Scientists carefully alter one variable in an experiment to see what happens. An architect might alter building plans to add more windows. A musician alters a familiar melody to create an interesting variation.

When something cannot be changed, we call it unalterable. The past is unalterable: you can learn from it, but you cannot change what already happened.