From The Airport Wiki

The A318, also known as the "Mini-Airbus" or "baby 'bus", is the smallest member of the A320 family. It originated from the AVIC1 and AIA cooperation program AE31X[13][14]. During development, it was known as the "A319M3," thus indicating its history as a direct derivative of the A319. "M3" indicates "minus three fuselage frames." The aircraft is six metres shorter and 14 tonnes lighter than the A320. Pilots who are trained on the other A320 variants may fly the A318 with no further certification, since it features the same type rating as its sister aircraft.

The A318 has a passenger capacity of 109 in a two-class configuration. It is intended to replace early Boeing 737 and Douglas DC-9 models, though it is also a rival to the 737-600. Boeing also offered their 717 aircraft as a competitor, although it was suitable primarily for regional routes and did not have the A318's range capabilities.

The A318 is available with a variety of different maximum take-off weights (MTOW) ranging from a 59 tonne, 2,750 km (1,500 nautical mile) base model to a 68 tonne, 6,000 km (3,250 nautical mile) version. The lower MTOW enables it to operate regional routes economically whilst sacrificing range and the higher MTOW allows it to complement other members of the A320 family on marginal routes. The lighter weight of the A318 gives it an operating range 10% greater than the A320, allowing it to serve some routes that the A320 would be unable to: London-Jerusalem and Singapore-Tokyo, for instance. Its main use for airlines, however, is on short, low-density hops between medium cities.