Word represents a word in Read, Find and related functions.
Words are defined to be sequences of characters that lie between separators. The separators are strings given as the settings for WordSeparators and RecordSeparators.
The default is for words to be delimited by "white space" consisting of spaces, tabs and newlines.
See The Mathematica Book: Section 2.10.3 and Section 2.11.8.
See also: Record.