14. The system of claim 13 , wherein generating the ranking of the plurality of advertisements comprises ordering the plurality of advertisements according to respective semantic distances between the first location of the semantic space of the first term and respective locations of the plurality of advertisements in the semantic space.
13. The system of claim 8 , wherein the operations further comprise: generating a ranking of a plurality of advertisements that are located within a threshold semantic distance to the first location in the semantic space of the first term; and determining that the advertisement is one of the highest-ranked advertisements in the ranking of the plurality of advertisements.