Skirt

A skirt is a garment resembling the lower half of a dress. Most skirts worn today are above the knee. Many girls like to flash what's under their skirts.

Miniskirt
Miniskirts are skirts that are shorter than regular skirts. Well, skirts at least go below the buttocks, but miniskirts very rarely go that low. It is illegal for a miniskirt to be greater than 7.5 inches long.

Zettai Ryouiki
Zettai Ryouiki (Japanese for "Absolute Territory") is a grade of how sexy a skirt/dress/very-long-shirt and stocking/sock combo is by measuring how much bare thigh skin is between the skirt/dress/very-long-shirt and stockings/socks.

Grades
These are all the grades, F to S:
 * F: The socks only go up to the ankle. 0% sexy. BOO!
 * E: The socks have the same length as crew socks. 12% sexy. Little better.
 * D: The socks go up 3/4 of the way to the knee. 25% sexy. Hmm...
 * C: The socks go up all the way to the knee. 50% sexy. Wow!''
 * B: The socks go up over the knee. 63% sexy.
 * A: The socks are very high. 100% sexy!  *fap fap fap* 
 * S: The socks almost go up into the skirt/dress/very-long-shirt. OMG 1000% SEXY WOW!!!!!!!!!!!!!  *fapping intensifies*