This part may be known locally in the Northeastern U.S. by that name. Technically, as it is used commonly with 3/4-inch copper tubing, installers might also ask for a 3/4 by 1/8 FPT by 3/4 solder tee; although they come in larger and smaller sizes for other tubing sizes.
A baseboard tee looks like a brass or forged copper solder elbow with a 1/8-inch female pipe tap at the curve. Air vents or plugs are screwed into the tapping to permit air removal or gauge service at a later date.
Go to http://www.hydronic.net in the series design course there to see the proper use of the tapping and fittings that insert into them.