Bouma is the unfortunate psycholinguistics noun for word shape as formed by the counterspaces, crossbars, ascenders, and descenders of a word's constituent letters.
It is named for the researcher Herman Bouma who introduced the effect in his 1973 paper titled "Visual Interference in the Parafovial Recognition of Initial and Final Letters of Words" (Vision Research, 13, 762-782).
Recognizing boumae (or boumas) is one of the key processes of reading letter-based languages. As our eye focuses our fovea on a point along text, our perceptual span is roughly 15 letters to the left and right (with all sorts of presumptions about type size and reading distance). Beyond that range, our parafovea is sending blurry boumae to the cerebral cortex, helping us to subconsciously plan our next saccade.
Since James Cattell first proposed bouma (though not by that name) recognition as the primary function of reading in 1886, many psycholinguists and typographers erroneously believed it to be the only method. (Thus much has been made of recognizable boumae in font design.) Many more recent studies have shown that bouma recognition is just a sizable part of a much more complex system that allows us to read.
Sources:
Phil Meggs, A History of Graphic Design
http://www.microsoft.com/typography/ctfonts/WordRecognition.aspx
http://www.themicrofoundry.com/ss_read1.html