The Airbus A318 is the smallest commercial jet in the A320 family.
Airbus designed the A318 as a shortened version of the A320, with a typical two-class capacity of about 107 passengers. Its compact size suited thinner routes and airports with shorter runways, although sales were much lower than those of the larger A319, A320 and A321.
The A318 first flew on 15 January 2002 and entered airline service with Frontier Airlines in 2003. It retained the A320 family’s common cockpit philosophy and fly-by-wire control system, allowing substantial pilot commonality with the other variants.
The A220-100 is smaller than the A318 in many seating layouts, but it belongs to the A220 family, not the A320 family. That family distinction is why the A318 is the canonical answer to this question.