Market share refers to the portion or percentage of total sales within a market that is captured by a particular company or product. It is a key indicator of a company's competitiveness and performance relative to its competitors in a specific industry or sector. Market share is calculated by dividing a company's sales revenue or unit sales volume by the total sales revenue or unit sales volume of the entire market and then multiplying by 100 to express it as a percentage.

For example, if Company A sells $10 million worth of products in a market with total sales of $100 million, Company A's market share would be 10% (10 million divided by 100 million, multiplied by 100). 

Market share is an important metric for businesses as it can indicate their relative strength or weakness in the market, influence investment decisions, and help in strategic planning and competitive analysis. A higher market share generally suggests that a company has a stronger presence and competitive advantage in the market, while a lower market share may indicate the need for improvement or adjustment in the company's strategies.