Dictionary.com
definitions

mitigate

[mit-i-geyt]
verb (used with object), mit·i·gat·ed, mit·i·gat·ing.
  1. to lessen in force or intensity, as wrath, grief, harshness, or pain; moderate.
  2. to make less severe: to mitigate a punishment.
  3. to make (a person, one's state of mind, disposition, etc.) milder or more gentle; mollify; appease.
verb (used without object), mit·i·gat·ed, mit·i·gat·ing.
  1. to become milder; lessen in severity.

Origin of mitigate

1375–1425; late Middle English mitigaten < Latin mītigātus (past participle of mītigāre to calm, soften, soothe), equivalent to mīt(is) mild, soft, gentle + -ig- (combining form of agere to do, cause to do, make) + -ātus -ate1
Related formsmit·i·ga·ble [mit-i-guh-buh l] /ˈmɪt ɪ gə bəl/, adjectivemit·i·gat·ed·ly, adverbmit·i·ga·tion, nounmit·i·ga·tive, mit·i·ga·to·ry [mit-i-guh-tawr-ee, -tohr-ee] /ˈmɪt ɪ gəˌtɔr i, -ˌtoʊr i/, adjectivemit·i·ga·tor, nounnon·mit·i·ga·tive, adjectivenon·mit·i·ga·to·ry, adjectiveo·ver·mit·i·gate, verb, o·ver·mit·i·gat·ed, o·ver·mit·i·gat·ing.un·mit·i·ga·ble, adjectiveun·mit·i·ga·tive, adjective
Can be confusedmilitate mitigate (see usage note at the current entry)

Usage note

Mitigate, whose central meaning is “to lessen” or “to make less severe,” is sometimes confused with militate, which means “to have effect or influence; weigh on.” This mix-up often occurs in the use of the phrase mitigate against, as follows: This criticism in no way mitigates (read militates ) against your going ahead with your research. Although this use of mitigate occasionally occurs in edited writing, it is rare and is widely regarded as an error.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for mitigator

mitigate

verb
  1. to make or become less severe or harsh; moderate
Derived Formsmitigable (ˈmɪtɪɡəbəl), adjectivemitigation, nounmitigative or mitigatory, adjectivemitigator, noun

Word Origin for mitigate

C15: from Latin mītigāre, from mītis mild + agere to make

usage

Mitigate is sometimes wrongly used where militate is meant: his behaviour militates (not mitigates) against his chances of promotion
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for mitigator

mitigate

v.

early 15c., "relieve (pain)," from Latin mitigatus, past participle of mitigare "soften, make tender, ripen, mellow, tame," figuratively, "make mild or gentle, pacify, soothe," ultimately from mitis "gentle, soft" (from PIE *mei- "mild") + root of agere "do, make, act" (see act). First element is from PIE root *mei- "soft, mild." Related: Mitigated; mitigating; mitigates.

Online Etymology Dictionary, © 2010 Douglas Harper
mitigator in Medicine

mitigate

[mĭt′ĭ-gāt′]
v.
  1. To moderate in force or intensity.
Related formsmit′i•ga′tion n.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.