Source : Webster's Revised Unabridged Dictionary (1913)

Spirometer \Spi*rom"e*ter\, n. [L. spirare to breathe + -meter.]
   An instrument for measuring the vital capacity of the lungs,
   or the volume of air which can be expelled from the chest
   after the deepest possible inspiration. Cf. {Pneumatometer}.

Source : WordNet®

spirometer
     n : a measuring instrument for measuring the vital capacity of
         the lungs