The Collaborative International Dictionary of English v.0.48:
Commensurable \Com*men"su*ra*ble\, a. [L. commensurabilis; pref.
com- + mensurable. See Commensurate, and cf.
Commeasurable.]
Having a common measure; capable of being exactly measured by
the same number, quantity, or measure. --
Com*men"su*ra*ble*ness, n.
[1913 Webster]
Commensurable numbers or Commensurable quantities
(Math.), those that can be exactly expressed by some
common unit; thus a foot and yard are commensurable, since
both can be expressed in terms of an inch, one being 12
inches, the other 36 inches.
Numbers commensurable in power, or Quantities
commensurable in power, those whose squares are
commensurable.
[1913 Webster]