14. 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: organizing advertisements according to their meaning into a lexicon, the lexicon defining elements of a semantic space and represented by a network of interconnected meanings; receiving an input term; identifying a first point in the semantic space associated with the input term based on an interpretation of potential meanings of the input term; determining a first semantic sub-space that includes all of the elements of the semantic space within a predetermined radius of the first point; identifying, in the first semantic sub-space, an advertisement proximate in semantic distance to the input term, wherein the semantic distance identifies how closely related the advertisement is to the input term; and providing the advertisement in response to the input term.
14. 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: organizing advertisements according to their meaning into a lexicon, the lexicon defining elements of a semantic space and represented by a network of interconnected meanings; receiving an input term; identifying a first point in the semantic space associated with the input term based on an interpretation of potential meanings of the input term; determining a first semantic sub-space that includes all of the elements of the semantic space within a predetermined radius of the first point; identifying, in the first semantic sub-space, an advertisement proximate in semantic distance to the input term, wherein the semantic distance identifies how closely related the advertisement is to the input term; and providing the advertisement in response to the input term. 17. The system of claim 14 , wherein the operations further comprise: receiving an indication of a meaning to associate with each advertisement from an advertiser associated with each advertisement; receiving a cost-per-impression amount associated with the meaning from the advertiser; and charging each advertiser the cost-per-impression amount associated with the meaning if the advertisement is provided in response to an input term.