NOUN (3)

1. minimal (or marginal) resources for subsisting;
- Example: "social security provided only a bare subsistence"

2. a means of surviving;
- Example: "farming is a hard means of subsistence"

3. the state of existing in reality; having substance;

The Collaborative International Dictionary of English v.0.48:

Subsistence \Sub*sist"ence\, n. [Cf. F. subsistance, L.
   subsistentia.]
   1. Real being; existence.
      [1913 Webster]

            Not only the things had subsistence, but the very
            images were of some creatures existing.
                                                  --Stillingfleet.
      [1913 Webster]

   2. Inherency; as, the subsistence of qualities in bodies.
      [1913 Webster]

   3. That which furnishes support to animal life; means of
      support; provisions, or that which produces provisions;
      livelihood; as, a meager subsistence.
      [1913 Webster]

            His viceroy could only propose to himself a
            comfortable subsistence out of the plunder of his
            province.                             --Addison.
      [1913 Webster]

   4. (Theol.) Same as Hypostasis, 2. --Hooker.
      [1913 Webster]

WordNet (r) 3.0 (2006):

subsistence
    n 1: minimal (or marginal) resources for subsisting; "social
         security provided only a bare subsistence"
    2: a means of surviving; "farming is a hard means of
       subsistence"
    3: the state of existing in reality; having substance