1. A computer-implemented method, comprising: accessing textual content that is included in one or more electronic documents; extracting one or more phrases from the textual content, the extracting including identifying single words or groups of words in the textual content that relate to a common concept; determining, for each given extracted phrase, a caps first ratio that specifies a frequency with which one or more words included in the given extracted phrase are capitalized within the one or more electronic documents; determining, by one or more computers and for each given extracted phrase, whether the given extracted phrase has two or more alternative meanings, the given extracted phrase being determined to have two or more alternative meanings when the caps first ratio of the given extracted phrase does not meet a specified caps first ratio threshold, and the given extracted phrase being determined to not have two or more alternative meanings when the caps first ratio meets the specified caps first ratio threshold; identifying a set of one or more unambiguous terms from the extracted phrases based on the determination; identifying, based on the set of one or more unambiguous terms, supplemental content related to the textual content; and distributing, over a communications network and to a user device, data that initiate presentation of the supplemental content at a display of the user device.