ménage à trois
[translated as “household of three”]
-noun
a domestic arrangement in which three people having sexual relations occupy the same household
In contemporary usage, the meaning of the term has been extended to mean any living relationship between three people, whether or not sex is involved, but because it has also been extended to refer to the actual sexual act between three people, otherwise known as a threesome, the term retains its suggestive quality.

neu∙ro∙sci∙ence
-noun
the field of study encompassing the various scientific disciplines dealing with the structure, development, function, chemistry, pharmacology, and pathology of the nervous system
(Source: neurolinguist)
l’esprit de l’escalier
(or l’esprit d’escalier)
[translated as “staircase wit”]
-verb
the act of thinking of a clever comeback when it is too late to deliver it
The phrase can be used to describe a riposte to an insult or any witty remark that comes to mind too late to be useful, after one has left the scene of the encounter. The phenomenon is usually accompanied by a feeling of regret at not having thought of the retort when it was most needed or suitable.
mamihlapinatapai
(sometimes spelled mamihlapinatapei)
-noun [origin: the Yaghan language of Tierra del Fuego]
a look shared by two people with each wishing that the other will initiate something that they both desire but which neither one wants to start.
The word consists of the reflexive/passive prefix ma- (mam- before a vowel), the root ihlapi, which means to be at a loss as what to do next, the stative suffix -n, an achievement suffix -ata, and the dual suffix -apai, which in composition with the reflexive mam- has a reciprocal sense.
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punctuated equilibrium
(also called punctuationalism)
-noun
theory of, Biology. a hypothesis holding that the evolution of species proceeds in a characteristic pattern of relative stability for long periods of time interspersed with much shorter periods during which many species become extinct and new species emerge