Market share is the slice of all sales one company holds. Gain market share.
It can rise while sales fall, since it is a proportion of a market that may be shrinking. Market share bought by cutting prices is the easiest kind to win and the hardest to keep.
market share is a measure.
Market plus share, from business writing of the twentieth century.
From market + share.
Etymology from Wiktionary, CC BY-SA
Practised in Marketing & Advertising