The Collaborative International Dictionary of English v.0.48:

Similiter \Si*mil"i*ter\, n. [L., in like manner.] (Law)
   The technical name of the form by which either party, in
   pleading, accepts the issue tendered by his opponent; --
   called sometimes a joinder in issue.
   [1913 Webster]

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

SIMILITER, pleading. When the defendant's plea contains a direct
contradiction of the declaration, and concludes with referring the matter to
be tried by a jury of the country, the plaintiff must do so too; that is, he
must also submit the matter to be tried by a jury, without offering any new
answer to it, and must stand or fall by his declaration. Co. Litt. 126 a. In
such case, he merely replies that as the defendant has put himself upon the
country, that is, has submitted his cause to be tried by a jury of the
country, he, the plaintiff, does so likewise, or the like. Hence this sort
of replication is called a similiter, that having been the effective word
when the proceedings were in Latin. 1 Chit. Pl. 549; Arch. Civ. Pl. 250. See
Steph. Pl. 255; 2 Saund. 319, b; Cowp. 407; 1 Str. Rep. 551; 11 S. & R. 32.