Prefixes from Latin

A prefix is a small sub-unit of language: it is a letter or group of letters placed at the beginning of a word to form a new word with a different meaning from the original word.

Many prefixes in English derive from the Classical Languages, i.e., from Greek and Latin. Many, though not all, of them were prepositions in Greek or Latin, and many, though not all, also functioned as prefixes in those languages. Here is a list of the more common prefixes which have a Latin origin. (For a list of the more common prefixes which have a Greek origin see Prefixes from Greek.) The first column lists the basic form of the preposition; the last column lists some of the different ways in which some prepositions appear. For example, 'accusation' is formed from the preposition ad- with the noun causa ('cause'); before the letter '-c-' ad- takes the form ac- by assimilation of sounds. In 'ascend', the ad- is reduced to a simple a-, presumably because adscend produces a clumsy consonant string.

Prefix	meaning	examples	other forms listed in OED
ab	away, from	abdicate, abduct	reduced to a- before p-, m-, and v-; au- before f-; abs- before c-, t-.
ad-	to, towards, near	adjacent, admission, admixture	before c, f, g, l, n, p, q, r, s, t, assimilated to ac-, af-, ag-, al-, an, ap-, ac-, ar-, as-, at-; reduced to a- before sc, sp, st
ante-	before	antechamber, antenatal
circum-	about, around, surrounding	circumference, circumlocution, circumnavigate
co-	together with, joint, mutual	coauthor, coeval, cohabit
com-, con-	with, together, altogether	compress, concentrate, consolidate	assimilated before l, r; before h, gn and vowels; con- before all other consonants
contra-	against, opposite	contradict, contraflow	in words from modern French, modified to contre; from Old French, counter-
de-	away, down	degrade, depress
dis-	not, reversal, separation	dissimilar, disembark, disband	assimilated before f; often reduced to di-
ex-	out of, from, former	exhale, export, ex-serviceman	often reduced to e-; assimilated to f.
extra-	outside, beyond the scope of	extra-curricular, extraordinary
in-1	not	incoherent, incombustible	sometimes assimilated to il-, im-, ir-; sometimes reduced to i- before gn (e.g. ignorant)
in-2	in, into, towards	incursion, infiltrate	sometimes assimilated to il-, im-, ir-; sometimes em-, en- in words from French
infra-	below, beneath	infrared, infrastructure
inter-	between, among, together	interact, interject, international	only in entertain, enterprise does the French form entre- survive
intra-	within, on the inside	intramural, intravenous
intro-	in, into, inwards	introspection, introvert
non-	not	non-combatant, nonconformist, nonentity
ob-	to, towards, against	object, oblige, obnoxious	assimilated to oc- before c-, of- before f-, op- before p-; reduced to o before m-
post-	after	postgraduate, posthumous, postscript
pre-	before	predecessor, predict, pre-empt II you may occasionally see the purer Latin form prae- in older English
pro-	in favour of, acting as a substitute for	pro-choice, pro-vicechancellor
re-	once more, return to a previous state	re-entry, recharge	occasionally red- before vowels
retro-	back, backwards, behind	retroactive, retrograde, retrospective
semi-	half, partly	semicircle, semi-skilled
sub-	below, lower, subordinate	submarine, subhuman, subdivision, subjugate
super-	over, above, outstanding	superego, supernatural
trans-	across, beyond, into another state	transfer, transgress, transfusion