Etymology
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midst (n.)

"the middle; an interior or central part, point, or position," c. 1400, from Middle English middes (mid-14c.), from mid (adj.) + adverbial genitive -s. The unetymological -t is perhaps on model of superlatives (compare against).

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*medhyo- 
Proto-Indo-European root meaning "middle." Perhaps related to PIE root *me- (2) "to measure."

It forms all or part of: amid; intermediate; mean (adj.2) "occupying a middle or intermediate place;" medal; medial; median; mediate; medieval; mediocre; Mediterranean; medium; meridian; mesic; mesial; meso-; meson; Mesopotamia; Mesozoic; mezzanine; mezzo; mezzotint; mid (prep., adj.); middle; Midgard; midriff; midst; midwife; milieu; minge; mizzen; moiety; mullion.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit madhyah, Avestan madiya- "middle," Greek mesos, Latin medius "in the middle, between; from the middle," Gothic midjis, Old English midd "middle," Old Church Slavonic medzu "between," Armenian mej "middle."
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in medias res 

Latin, literally "in the midst of things," from medias, accusative fem. plural of medius "middle" (see medial (adj.)) + accusative plural of res "a thing" (see re). From Horace, in reference to narrative technique:

Semper ad eventum festinat, et in medias res,
Non secus ac notas auditorem rapit (etc.)
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interim (n.)
"time intervening," 1560s, from Latin interim (adv.) "in the meantime, meanwhile," originally "in the midst of that," from inter "between" (from PIE *enter "between, among," comparative of root *en "in") + im, ancient adverb from stem of pronoun is "this, that." As an adjective from c. 1600.
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oasis (n.)

"fertile spot in a desert, where there is a spring or well and more or less vegetation," originally in reference to the Libyan desert, 1610s, from French oasis (18c.) and directly from Late Latin oasis, from Greek oasis, probably from Hamitic (compare Coptic wahe, ouahe "oasis," properly "dwelling place," from ouih "dwell"). The same Egyptian source produced Arabic wahah. Figurative sense of "any fertile place in the midst of a waste" is by 1800.

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amidst (prep.)

a variant of amid (q.v.) with adverbial genitive -s and unetymological -t. Amidde became amyddes (13c.) and acquired the -t from mid-15c., probably by association with superlatives in -st; the pattern also yielded amongst, against, betwixt, whilst, also archaic alongst (13c.-17c.).

There is a tendency to use amidst more distributively than amid, e.g. of things scattered about, or a thing moving, in the midst of others. [OED]
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among (prep.)

"in, in the midst of," early 12c., from Old English onmang, in late Old English sometimes amang, a contraction of ongemang "among, during," from phrase on gemang, literally "in the crowd or company (of)," from on (see a- (1)) + gemengan "to mingle," from Proto-Germanic *mangjan "to knead together," which is perhaps from a nasalized form of PIE root *mag- "to knead, fashion, fit." The collective prefix ge- was dropped 12c. leaving onmong, amang, among. Compare Old Saxon angimang "among, amid;" Old Frisian mong "among."

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explanation (n.)

"an act of explaining; a meaning or interpretation assigned," late 14c., explanacioun, from Latin explanationem (nominative explanatio) "an explanation, interpretation," noun of action from past-participle stem of explanare "to make plain or clear, explain," literally "make level, flatten," from ex "out" (see ex-) + planus "flat" (from PIE root *pele- (2) "flat; to spread").

An individual fact is said to be explained, by pointing out its cause, that is, by stating the law or laws of causation of which its production is an instance. Thus. a conflagration is explained, when it is proved to have arisen from a spark falling into the midst of a heap of combustibles. [J.S. Mill, "Logic"]
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Mediterranean 

"the sea between southern Europe and northern Africa," 1590s, earlier Mediterranie (c. 1400), from Late Latin Mediterraneum mare "Mediterranean Sea" (7c.), from Latin mediterraneus "midland, surrounded by land, in the midst of an expanse of land" (but in reference to the body of water between Europe and African the sense probably was "the sea in the middle of the earth"); from medius "middle" (from PIE root *medhyo- "middle") + terra "land, earth" (from PIE root *ters- "to dry").

The Old English name was Wendel-sæ, so called for the Vandals, Germanic tribe that settled on the southwest coast of it after the fall of Rome. The noun meaning "a person of Mediterranean race" is by 1888.

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mid (adj.)

"middle; being the middle part or midst; being between, intermediate," Old English mid, midd from Proto-Germanic *medja- (source also of Old Norse miðr, Old Saxon middi, Old Frisian midde, Middle Dutch mydde, Old High German mitti, German mitte, Gothic midjis "mid, middle"), from PIE root *medhyo- "middle."

By late Middle English probably felt as a prefix only, and now surviving in English only as a prefix (mid-air, midstream, etc.). Prefixed to months, seasons, etc. from late Old English. As a preposition, "in the middle of, amid" (c. 1400) it is from in midde or a shortened form of amid (compare midshipman) and sometimes is written 'mid.

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