NOUN (4)

1. social status or position conferred by a system based on class;
- Example: "lose caste by doing work beneath one's station"

2. (Hinduism) a hereditary social class among Hindus; stratified according to ritual purity;

3. a social class separated from others by distinctions of hereditary rank or profession or wealth;

4. in some social insects (such as ants) a physically distinct individual or group of individuals specialized to perform certain functions in the colony;

The Collaborative International Dictionary of English v.0.48:

Caste \Caste\, n. [Pg. casta race, lineage, fr. L. castus pure,
   chaste: cf. F. caste, of same origin.]
   1. One of the hereditary classes into which the Hindu are
      divided according to the laws of Brahmanism.
      [1913 Webster]

   Note: The members of the same caste are theoretically of
         equal rank, and same profession or occupation, and may
         not eat or intermarry with those not of their own
         caste. The original are four, viz., the Brahmans, or
         sacerdotal order; the Kshatriyas, or soldiers and
         rulers; the Vaisyas, or husbandmen and merchants; and
         the Sudras, or laborers and mechanics. Men of no caste
         are Pariahs, outcasts. Numerous mixed classes, or
         castes, have sprung up in the progress of time.
         [1913 Webster]

   2. A separate and fixed order or class of persons in society
      who chiefly hold intercourse among themselves.
      [1913 Webster]

            The tinkers then formed an hereditary caste.
                                                  --Macaulay.
      [1913 Webster]

   To lose caste, to be degraded from the caste to which one
      has belonged; to lose social position or consideration.
      [1913 Webster]

WordNet (r) 3.0 (2006):

caste
    n 1: social status or position conferred by a system based on
         class; "lose caste by doing work beneath one's station"
    2: (Hinduism) a hereditary social class among Hindus; stratified
       according to ritual purity
    3: a social class separated from others by distinctions of
       hereditary rank or profession or wealth
    4: in some social insects (such as ants) a physically distinct
       individual or group of individuals specialized to perform
       certain functions in the colony

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "caste":
   ashram, blood, body, bracket, branch, category, clan, class,
   colony, commonwealth, commune, community, condition, division,
   echelon, economic class, endogamous group, estate, extended family,
   family, footing, gens, grade, group, grouping, head, heading,
   hierarchy, kin, kinship group, label, level, moiety,
   nuclear family, order, phratria, phratry, phyle, pigeonhole, place,
   position, power structure, precedence, predicament, race, rank,
   rate, rating, rubric, section, sept, set, settlement, social class,
   society, sphere, stage, standing, station, status, strain, stratum,
   subcaste, subdivision, subgroup, suborder, title, totem