Illustrated Glossary of Organic Chemistry

Lewis acid-base adduct (Lewis acid-base complex): A molecule formed by the bonding of a Lewis acid with a Lewis base, without simultaneous loss of a leaving group. (In other words the Lewis acid-Lewis base reaction cannot be a substitution reaction.) A Lewis acid-Lewis base adduct (or complex) is usually depicted with a raised dot between the Lewis acid and Lewis base components (Lewis acid.Lewis base), or (less commonly) an arrow indicating the electron donation from the Lewis base to the Lewis acid (Lewis baseLewis acid).

Reaction of borane (BH3;  a Lewis acid) with THF (a Lewis base) occurs when an oxygen lone pair forms an oxygen-boron bond. The reaction product is a Lewis acid-Lewis base adduct, because all of the reactant atoms are part of the product; there is no leaving group. The product can be abbreviated as BH3.THF or BH3THF.

Reaction of iodomethane (CH3I; a Lewis acid) with hydroxide ion (HO-; a Lewis base) to form methanol is not an example of Lewis acid-Lewis base adduct formation. Instead, it is an SN2 reaction because iodide ion (the leaving group) departs.