Technically the word umlaut refers to the sound change in a front vowel such as a, o or u when the speaker is anticipating a following back vowel. Diæresis refers to the mark itself for whatever purpose it is used. Use of the word umlaut to refer to the mark is probably a little sloppy, but the word diæresis sounds too much like some sort of unspeakable and extremely unpleasant disease to the native English speaker and is therefore not as widely used as it should be.