The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

spirometer
    n 1: a measuring instrument for measuring the vital capacity of
         the lungs