Plain English Meaning
A standard, straightforward takeoff from a regular runway in ordinary conditions — no crosswind to fight, no short field to worry about, no soft surface, no obstacles to clear. It is the baseline takeoff every other type is compared to.
Full Definition
A takeoff in which the airplane is headed directly into the wind, or with the wind less than 10° off the nose, departing from a paved runway of adequate length and surface, with no obstructions along the takeoff path, at or near sea level, and using standard takeoff and climb procedures.