The Collaborative International Dictionary of English v.0.48:

Socage \Soc"age\, n.[From Soc; cf. LL. socagium.] (O.Eng. Law)
   A tenure of lands and tenements by a certain or determinate
   service; a tenure distinct from chivalry or knight's service,
   in which the obligations were uncertain. The service must be
   certain, in order to be denominated socage, as to hold by
   fealty and twenty shillings rent. [Written also soccage.]
   [1913 Webster]

   Note: Socage is of two kinds; free socage, where the
         services are not only certain, but honorable; and
         villein socage, where the services, though certain,
         are of a baser nature. --Blackstone.
         [1913 Webster]

WordNet (r) 3.0 (2006):

socage
    n 1: land tenure by agricultural service or payment of rent; not
         burdened with military service

Bouvier's Law Dictionary, Revised 6th Ed (1856):

SOCAGE, Eng. law. A tenure of lands by certain inferior services in
husbandry, and not knight's service, in lieu of all other services. Litt.
sect. 117.