The Collaborative International Dictionary of English v.0.48:

A mensa et thoro \A men"sa et tho"ro\ [L., from board and bed.]
   (Law)
   A kind of divorce which does not dissolve the marriage bond,
   but merely authorizes a separate life of the husband and
   wife. --Abbott.
   [1913 Webster]

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

A MENSA ET THORO, from bed and board.  A divorce a mensa et thoro, is rather
a separation of the parties by act of law, than a dissolution of the
marriage.  It may be granted for the causes of extreme cruelty or desertion
of the wife by the husband. 2 Eccl. Rep. 208.  This kind of divorce does not
affect the legitimacy of children, nor authorize a second marriage. V. A
vinculo matrimonii; Cruelty Divorce.