The extent to which a product is recognized and bought by customers in a particular market
Market penetration is one of the four growth strategies as defined by Ansoff. Market penetration occurs when a company enters/penetrates a market with current products. The best way to achieve this is by gaining competitors' customers (part of their market share). Other ways include attracting non-users of your product or convincing current clients to use more of your product/service (by advertising etc)