If $x$ and $y$ are elements of a poset, then their meet, or infimum, is an element $x \wedge y$ of the poset such that:

$x \wedge y \leq x$ and $x \wedge y \leq y$;

if $a \leq x$ and $a \leq y$, then $a \leq x \wedge y$. Such a meet may not exist; if it does, then it is unique.

In a proset, a meet may be defined similarly, but it need not be unique. (However, it is still unique up the natural equivalence in the proset.)

The above definition is for the meet of two elements of a poset, but it can easily be generalised to any number of elements. It may be more common to use ‘meet’ for a meet of finitely many elements and ‘infimum’ for a meet of (possibly) infinitely many elements, but they are the same concept. The meet may also be called the minimum if it equals one of the original elements.

A poset that has all finite meets is a meet-semilattice. A poset that has all infima is an inflattice.