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In non-standard analysis, a monad (also called halo[1]) [2] is the set of points infinitesimally close to a given point.

Given a hyperreal number x in R, the monad of x is the set

${\displaystyle {\text{monad}}(x)=\{y\in \mathbb {R} ^{*}\mid x-y{\text{ is infinitesimal}}\}.}$

If x is finite (limited), the unique real number in the monad of x is called the standard part of x.