in-2 Also il-, im-, and ir-.
[English in or Latin in-.]
Words in in- that derive from Latin are to various degrees figurative; sometimes the prefix gives additional force or emphasis. Examples include incantation, incarcerate, indoctrinate, induce, infect, influx, and inundate. In other cases the prefix derives from English in, as with income, inlet, insight, and intake; with these the literal sense of in is more often retained.
A distinction between this and the previous sense is not always obvious: immense looks as though it has in- as an intensifier, but actually it derives from Latin in- in the sense of ‘not’, plus mensus, measured. Similarly, inflammable looks as though it might mean ‘not capable of burning’, but it really means ‘easily set on fire' (this is such a potentially disastrous confusion that flammable has replaced it in safety instructions).
The Latin form became en- in French (see en-1), and many English words either now contain that spelling, or have done so in the past: enable used to be spelled inable; entail was once intail. Some words can exist in both forms: inquire is the usual spelling in the US, while in Britain enquire is more common. Sometimes differences in usage exist: insure usually means ‘protect oneself by insurance’, while ensure means ‘make sure’. In other cases, one or the other form predominates, as enclose does over inclose.
The prefix is spelled il- before stems beginning in l (illuminate, illustrate), im- before b, m, and p (imbibe, immerse, impersonate), and ir- before r (irradiate, irrigate).