The measure is the number of characters in a single line of a column of text. HTML doesn’t have a concept of columns per se, instead text is held within boxes.

[…]

When typographers set the measure and text size for printed media, those dimensions are fixed and unchangeable in their physical manifestation. In this regard, the Web as viewed on-screen is fundamentally different to print because the medium is far more under the control of your readers. In particular, if your reader wishes to change the text size or the dimensions of the ‘page’, he can do.

Source: http://webtypography.net/2.1.2