constancy

(noun) the quality of being enduring and free from change or variation. Synonyms: stability. Examples: "Early mariners relied on the constancy of the trade winds."

(noun) (psychology) the tendency for perceived objects to give rise to very similar perceptual experiences in spite of wide variations in the conditions of observation. Synonyms: perceptual constancy.

(noun) faithfulness and dependability in personal attachments (especially sexual fidelity).