diluent

1. Ingredient in a medicinal preparation that lacks pharmacologic activity but is pharmaceutically necessary or desirable. In tablet or capsule dosage forms, this may be lactose or starch; it is particularly useful in increasing the bulk of potent drug substances whose mass is too small for dosage form manufacture or administration. May be a liquid for the dissolution of drug(s) to be injected, ingested, or inhaled. 2. Denoting that which dilutes; the diluting agent.

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dil·u·ent 'dil-yə-wənt n a diluting agent (as the vehicle in a medicinal preparation)

diluent adj making thinner or less concentrated by admixture: DILUTING

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dil·u·ent (dilґu-ənt) 1. causing dilution. 2. an agent that dilutes or renders less potent or irritant.