The Collaborative International Dictionary of English v.0.48:

Subjunction \Sub*junc"tion\, n. [See Subjunctive.]
   1. Act of subjoining, or state of being subjoined.
      [1913 Webster]

   2. Something subjoined; as, a subjunction to a sentence.
      [1913 Webster]

WordNet (r) 3.0 (2006):

subjunction
    n 1: the act of supplementing [syn: supplementation,
         subjunction, subjoining]