Lone Pair Definition:
A Lone Pair or non-bonding pair is one pair of unshared or nonbinding electrons on an atom.
Lone Pair Explained:
An easy way to identify the lone pair electrons is by examining the lewis dot structure of the atom. The structure shows the total number of valence electrons that comprise both the bonding electrons as well as the non-binding/lone pair electrons.
(However, note that not all nonbonding pairs of electrons are considered to be lone pairs. Examples of which include transition metals wherein the non-bonding pairs do not influence molecular geometry. This concept will be further explained in the valence shell electron pair repulsion theory (VSEPR theory) and the inert pair effect that explains the shapes of molecules.)