Definition of sesqui- in US English:

sesqui-

combining form

  • 1Denoting one and a half.

    ‘sesquicentenary’
    1. 1.1Chemistry (of a compound) in which a particular element or group is present in a ratio of 3:2 compared with another.
      ‘sesquioxide’

Origin

From Latin semi- (see semi-) + que ‘and’.