Market penetration refers to the percentage of a target market that uses a specific product or service. It can be measured as a company's sales as a percentage of the total sales for that product category. The document discusses various tactics for increasing market penetration, such as knowing the risks of entering new markets versus focusing on existing markets, creating barriers to entry for competitors, being unique in marketing approaches, diversifying product lines, and forming strategic alliances. It emphasizes using multiple tactics together for the most effective market penetration.