Lone pair

In chemistry, a lone pair is a valence electron pair which is not shared with another atom and is sometimes called a non-bonding pair.

Quotes

 * Valence electrons that are not used for bonding are called lone-pair electrons, or nonbonding electrons. … As a time-saving shorthand, nonbonding electrons are often omitted when drawing line-bond structures, but you still have to keep them in mind since they’re often crucial in chemical reactions.
 * John McMurry, Organic Chemistry 8th ed. (2012), Ch. 1 : Structure and Bonding.