Longman Dictionary of Contemporary English
The Collaborative International Dictionary
Em- \Em-\ A prefix. See En-.
Douglas Harper's Etymology Dictionary
word-forming element meaning "put in or into, bring to a certain state," sometimes intensive, from French assimilation of en- "in, into" (see en- (1)) to following labial stop (-b-, -p-, and often -m-), or from the same development in later Latin in- (to im-). "This rule was not fully established in spelling before the 17th c." [OED], but it is likely the pronunciation shift was in Old French and Middle English and spelling was slow to conform. Also a living prefix in English used to form verbs from adjectives and nouns ( embitter, embody).
in words such as emancipate, emerge, emit, emotion represents Latin ex- assimilated to following -m- (see ex-).
pre. The form taken by en- as it assimilates place of articulation before labial consonants (''b'', ''m'' and ''p'').