(Latin: in, into, within, inside, on, toward [il-, ir-, im-], in, into, etc.: involve, incur, invade; also, used intensively, as in the words inflame and inflammable, or without perceptible force.)

The in- changes or is assimilated to il- before l, as with "illuminate", to im- before b, as with "imbibe"; before m, as with "immediate"; before p, as with "implant"; and to ir- before r, as with "irrigate".

The form generally remains unassimilated in words formed in English; such as, inbreed.

Don't confuse this in-2, meaning "in, into, within", etc. with the Latin prefix in-1 meaning "not" nor with the prefix for English-origin words in-3 meaning "in, into; within".

irradiate (verb), irradiates; irradiated; irradiating
1. To expose someone to or to treat something with radiation or streams of particles.
2. To treat food with electromagnetic radiation or to kill microorganisms and slow down the process of ripening and gradual deterioration or rotting.
3. To make something brighter by shining light onto it.
4. To make something intellectually clear.
5. Etymology: "to cast beams of light upon"; from Latin irradiatus, irradiare, "to shine forth"; from in-, "into, in" + radiare, "to shine".
irrigate (verb), irrigates; irrigated; irrigating
1. To provide dry land with water by using pipes, ditches, or streams; to artificially water an area.
2. To clean a body wound or cavity with water or a medicated fluid.
3. Etymology: "to supply land with water"; from Latin irrigatus, irrigare, "to lead water to, to refresh, to flood"; from a form of in-, "into, in, on, upon" + rigare "to water, to moisten".
irrigation (s) (noun), irrigations (pl)
1. An act or process of causing water to flow by mechanical distribution; usually, on agricultural land with ditches, or artificial canals, etc.: "Farmers in dry areas use irrigation to grow food crops."
2. A medical system of washing out a part of the body; such as, a cavity or a wound in order to clean it for further medical treatment: "One example is colonic irrigation and another one is the irrigation of an open wound before the doctor proceeds with additional treatments."