humectant

adj. (about a substance) retaining moisture

A humectant is a hygroscopic substance. It is often a molecule with several hydrophilic groups, most often hydroxyl groups, but amines and carboxyl groups, sometimes esterified, can be encountered as well; the affinity to form hydrogen bonds with molecules of water is crucial here.Since hygroscopic substances absorb water from the air, they are frequently used in desiccation. When used as a food additive, the humectant has the effect of keeping the foodstuff moist. Humectants are sometimes used as a component of antistatic coatings for plastics. Humectants are also found in many cosmetic products where moisturization is desired, including treatments such as moisturizing hair conditioners.
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Noun
1. any substance that is added to another substance to keep it moist
(hypernym) substance, matter
(hyponym) propylene glycol, propanediol

v. damp, dampen, moisten

(n.)
A diluent drink or medicine.
  

(a.)
Diluent.