Market penetration refers to the saturation of a market (usually given as a number describing what percent of a group of potential users/buyers of a product or service is already using or buying that product/service.
A highly penetrated or saturated market (like the US standard TV set market, where most families have at least one TV set) has little room for direct growth. A less densely penetrated or saturated market (like the rural Chinese cell phone market) is one where the current levels of use/saturation is significantly less than the projected total possible amount of use/saturation, so a market with lower penetration rates has more room for growth.
Market penetration