The size of the racquet's string bed, measured in square inches. Larger heads (100-110 sq in) offer more power and a bigger sweet spot, forgiving off-center hits. Smaller heads (93-98 sq in) offer more control and precision. Most recreational players should use a larger head. Most pros use something in the 97-100 range. Using a tiny head because your favorite pro does is a common mistake.
"She switched from a 100 to a 98 square inch head for more control on her volleys. Took three weeks to adjust."