Market share is the percentage of sales (by value or volume) that a brand or company has for a specific product or category.
Note: Market share calculations can range from specific products (i.e. Vanilla flavored ready to drink iced coffee) to extremely broad industries (i.e. food and beverage). Be sure you are aware of which industry segment(s) a report includes.
Above is an image from IBIS World (an industry database) that shows the market share of the Car and Automobile Manufacturing Industry report.