(covers information from several alternate timelines)
Environmental engineering (or life support center) was a section aboard Constitution-class starships where all life support activities on the ship were controlled and monitored. This section was located in the primary hull.
In 2266, Bobby climbed a ladder into a shaft labeled "environmental engineering personnel only". (TOS: "Mudd's Women") Later that year, the Psi 2000 virus infected Hikaru Sulu climbed up a same shaft with the same labeling, while carrying a rapier. (TOS: "The Naked Now") Some time later, that year, Leonard McCoy stood in front of a similar sign. (TOS: "The Conscience of the King")
Kirk and Scalosians in the Enterprise's environmental engineering section.
In 2268, when the Scalosians took over the USS Enterprise, they installed a deep-freeze device in environmental engineering with the intention of putting the entire crew into suspended animation. (TOS: "Wink of an Eye")
In the alternate reality, this center was located (in the) aft of the warp nacelles. In 2259, during negotiations to return his crew to him, Khan Noonien Singh threatened to destroy this center if his request was denied. (Star Trek Into Darkness)