8. A system, comprising: one or more computers; and a storage device coupled to the one or more computers having instructions stored thereon which, when executed by the one or more computers, cause the one or more computers to perform operations 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.
8. A system, comprising: one or more computers; and a storage device coupled to the one or more computers having instructions stored thereon which, when executed by the one or more computers, cause the one or more computers to perform operations 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. 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.