"act of walking about and performing apparently purposive acts while in a state between sleeping and waking," 1825; see somnambulism + -ance.
Entries linking to somnambulance
somnambulism n.
1786, "walking in one's sleep or under hypnosis," from French somnambulisme, from Modern Latin somnambulus "sleepwalker," from Latin somnus "sleep" (from PIE root *swep- "to sleep") + ambulare "to walk" (see amble (v.)). The word emerged during the excitement over "animal magnetism" and won out over noctambulation.
A stack of related words came into English use early 19c.: somnambulance, somnambulation, etc. As a noun for "sleepwalker, one who walks in sleep," somnambulist (1783, Beilby Porteus, "Sermons on Several Subjects"); somnambule (1837, from French somnambule, 1690s); somnambulator (1803); somnambulant (1819). As adjectives, "of, pertaining to, or characteristic of sleepwalking," somnambulic (1819); somnambulistic (1817); somnambulous (1799); somnambulary (1827), somnambular (1820).
-ance
word-forming element attached to verbs to form abstract nouns of process or fact (convergence from converge), or of state or quality (absence from absent); ultimately from Latin -antia and -entia, which depended on the vowel in the stem word, from PIE *-nt-, adjectival suffix.
Latin present-participle endings for verbs stems in -a- were distinguished from those in -i- and -e-. Hence Modern English protestant, opponent, obedient from Latin protestare, opponere, obedire.
As Old French evolved from Latin, these were leveled to -ance, but later French borrowings from Latin (some of them subsequently passed to English) used the appropriate Latin form of the ending, as did words borrowed by English directly from Latin (diligence,absence).
English thus inherited a confused mass of words from French (crescent/croissant), and further confused it since c. 1500 by restoring -ence selectively in some forms of these words to conform with Latin. Thus dependant, but independence, etc.