During the last protest staged downtown by opponents of the Medical Area Total Energy Plant (MATEP), one of the demonstrators suggested that the issues surrounding the power plant were so complicated, somebody ought to write a MATEP dictionary. He wasn't joking. Over the past four years, as the project has skidded from hearing to decision and back to hearing, it has become almost necessary for those involved--on any side--to learn an entirely new vocabulary and become familiar with a cast of characters. They include:

MATEP: The Medical Area Total Energy Plant, a Harvard-planned project aimed at cutting energy costs for 13 institutions in the University's Medical Area.

DEQE: The state Department of Environmental Quality Engineering, the Commonwealth's agency charged with determining whether MATEP is environmentally safe and sound.

Harvard University: One of the most prestigious educational institutions in the world, Harvard decided to get involved in the power business. The institution that has financed the power plant stands to lose a lot of money if it isn't put into operation.

NOMATEP: A coalition of community groups in the Mission Hill and Brookline areas that argues that the plans for MATEP are environmentally unsound and is determined to block Harvard's project.

Mission Hill: A diverse ethnic neighborhood that borders the Harvard Medical Area, the residents of which object to the sight--and what may come out--of the smokestack of the Medical Area Total Energy Plant.

Brookline: An affluent, nearby suburb populated by many people with Harvard degrees and many professionals, some of whom have taken it on themselves to make sure the MATEP smokestack is not spewing noxious gases into their backyards.

Triple deckers: The three-story townhouses that the majority of people in Mission Hill inhabit. Harvard tore down many of these in order to make room for the power plant and is letting others fall into disrepair so it has an excuse to rip them down and build another hospital.

AHE: The Affiliated Hospitals Center, now known as the Brigham and Women's Hospital, which sits snugly in the shadow of the smokestack of the Medical Area Total Energy Plant. A $19 million, 680-bed facility, the hospital complex is the largest medical teaching facility in the world.

Diesel engines: Bloated versions of the motors they are putting in cars nowadays. The planners of MATEP chose to install six of these in the plant. They use no. 7 fuel oil and are extremely oil efficient, but are also acknowledged to be the worst polluting engines that can be used for cogeneration.

Cogeneration: Taking one main power source, and by capturing excess steam and heat, producing more than one type of energy. In the case of MATEP, the diesel engines are the main power source, producing electricity, chilled water and steam.

Nitrous oxides: What everybody is fighting about. Actually nitrogen dioxide, a brownish gas that causes respiratory irritation and disease when present at unsafe levels. Said to be particularly harmful to asthmatics and the very young and very old. People disagree about how much is unhealthy.

Hot spots: Particularly heavily trafficked areas, such as the Allston toll booth on the Massachusetts Turnpike and the entrance to the Callahan tunnel, which normally display a high level of nitrogen dioxide. Said to be a deadly mixture when combined with the nitrogen dioxide spewed from the smokestack of the Medical Area Total Energy Plant.

Among the Characters:

L. Edward Lashman: A very large, sternlooking man who once organized unions in the South and now handles the Medical Area Total Energy Plant for Harvard University. A man who has worked in and out of some of the toughest jobs in government, Lashman calls MATEP "the hardest thing I've ever worked on."

Joe B. Wyatt: Vice president for administration of Harvard University and the man ultimately responsible for the Medical Area Total Energy Plant. Wyatt inherited the power plant and is currently having his troubles setting utility rates for prospective users.

David Fierra: deputy commisisoner of the state Department of Environmental Quality Engineering, Fierra had the MATEP decision dumped in his lap when the DEQE commissioner disqualified himself from the case. An expert in air quality, Fierra is less skilled at administration and has trouble coming to quick decisions. He has the final say.

Thomas B. Bracken: One-time attorney for the Environmental Protection Agency, Bracken is attorney for the Brookline residents who oppose MATEP.

Daniel G. Partan: Member of the Brookline Selectmen's ad hoc committee on the power plant, Partan is perhaps the most technically versed of the plant's opponents. Joined on the committee by chairman Dr. John Hermos. Partan is a committed fighter who also knows what he's talking about.