What is the definition for the depth of a pant?

The depth of the hip portion, or the length from the waist to the top of the
inseam.
Usually the pants never have this written on the size plate. I am too slim
and legs
and butt are much longer compare to upper body...

Dear Feverish,

This measurement is called the "crotch depth." To measure, sit on a
flat surface (not a contoured chair--must be flat), and measure from
your natural waist where your side seam would be, to the flat surface .

this is your crotch depth. Since your mentioned your butt and legs,
you may need other measurements to get good-fitting pants. Try
measuring from your waist at the center front, through your legs to
center back, then sit down.
The tape will slide when you sit. This will give you your total crotch
measurement. Divide it in half, then add two inches to the back, and
subtract two inches from the front. This will give you the approximate
measurements for your pants.
There is only about 1/2 inch of ease in this area.

To re-draw longer legs, it's best to draw both inner and outer seams
from the hip. Take the existing patterns for back and front, and fold
them exactly in half. The crease is the center front and center back
of your leg. Measure out at the new length in both directions
from the center half the hem width. Mark. Draw a new side seam line
from the hip to your mark. Do the same for the inseam. NOTE: the
bottom of your pants must be altered the exact same amount on both
sides so that the resulting change won't twist.

Teri