2 1/2" hose thread is measured by the inside diameter of the male coupling.The most common thread type in use by fire departments today is NST or National Standard Thread. NST also goes by the the initials NH or National Hose. It can even be referred to as NHT or National Hose Thread but NST or NH is more common these days. NH = NST
The second most common thread is NPSH (also known as IPT). A female NPSH coupling can thread on to a male NPT and seal via gasket if the face of the NPT male is square. NPSH = IPT = SIPT and has the same TPI (threads per inch) as NPT. NPSH is the straight thread version of NPT which is tapered thread and common in non-fire department use.