1. A method comprising: receiving a request for one or more advertisements, wherein the request includes a first term; determining a first location in a semantic space of the first term, wherein the semantic space has a plurality of nodes, each node being associated with a different respective term, and wherein each of the plurality of nodes has a bond with at least one other node in the semantic space, and wherein the bonds between the nodes are assigned respective strengths; determining a semantic distance between the first location in the semantic space of the first term and a second location in the semantic space of a second term associated with an advertisement, including evaluating bonds between a first node associated with the first term and a second node associated with the second term including evaluating a sum of strengths of the bonds between the first node and the second node; obtaining an original cost-per-impression amount provided by an advertiser for the second term associated with the advertisement; modifying the original cost-per-impression amount for the second term provided by the advertiser based on the semantic distance in the semantic space between the first term and the second term, wherein a greater semantic distance results in a lower cost-per-impression amount and wherein a smaller semantic distance results in a higher cost-per-impression amount; providing the advertisement in response to the request; and charging the advertiser the modified cost-per-impression amount for the second term instead of the original cost-per-impression amount provided by the advertiser.