Merriam-Webster Collegiate Dictionary - attenuate

 I. adjective  Etymology: Middle English attenuat, from Latin attenuatus, past participle of attenuare to make thin, from ad- + tenuis thin — more at thin  Date: 15th century  1. reduced especially in thickness, density, or force  2. tapering gradually usually to a long slender point ~ leaves  II. verb  (-ated; -ating)  Date: 1530  transitive verb  1. to make thin or slender  2. to make thin in consistency ; rarefy  3. to lessen the amount, force, magnitude, or value of ; weaken  4. to reduce the severity, virulence, or vitality of an ~d virus  intransitive verb to become thin, fine, or less  • attenuation noun