Alternate text should be called Alternative text

The WC3 refers to it as Alternative text: http://www.w3.org/standards/webdesign/accessibility

Alternative Text for Images

Alt text is the classic example. Images should include equivalent alternative text in the markup/code.

If alt text isn't provided for images, the image information is inaccessible, for example, to people who cannot see and use a screen reader that reads aloud the information on a page, including the alt text for the visual image.

When equivalent alt text is provided, the information is available to everyone to people who are blind, as well as to people who turned off images on their mobile phone to lower bandwidth charges, people in a rural area with low bandwidth who turned off images to speed download, and others. It's also available to technologies that cannot see the image, such as search engines. 

Dictionary meanings for reference:
http://dictionary.reference.com/browse/alternate
http://dictionary.reference.com/browse/alternative