con·stant 
(kŏnstənt)
(kŏnstənt)adj.
1. Continually occurring; persistent: constant surveillance. See Synonyms at continual.
2. Regularly recurring: plagued by constant interruptions.
3. Unchanging in nature, value, or extent; invariable: a constant wind speed.
4. Steadfast in purpose, loyalty, or affection; faithful. See Synonyms at faithful.
n.
1. Something that is unchanging or invariable.
2.
a. A quantity assumed to have a fixed value in a specified mathematical context.
b. An experimental or theoretical condition, factor, or quantity that does not vary or that is regarded as invariant in specified circumstances.
[Middle English, from Old French, from Latin cōnstāns, cōnstant-, present participle of cōnstāre, to stand firm : com-, intensive pref.; see COM- + stāre, to stand; see stā- in the Appendix of Indo-European roots.]
constant·ly adv.