Those in which the final part of the original word is retained
Aeroplane > plane.
Omnibus > bus.
Those in which the medial part of the original word is retained.
Influenza > flu.
Refrigerator > fridge.
Those in which the final "s" of the original word is retained along with the inital or medial part of that word.
Pyjamas > jams.
Spectacles > specs.
Mathematics > maths.
Diggings > digs.