N.P.T. is the common abbreviation for National Pipe Tapered threads. NPT can also be called ANSI/ASME B1.20.1 pipe threads, but it is more commonly referred to as NPT. You may also see them referred to as NM and NF depending on whether they are a Male NPT (NM) or a Female NPT (NF).
Once you are familiar with NPT type threads, you will also notice that they have a slight taper to them. The taper is only 1° 47’ 24”, but you will notice that on a male fitting the thread is narrower near the tip and then wider at the shoulder. Also the female NPT fitting is wider at the mouth of the port and then narrower at the bottom. By the way, NPT threads are not interchangeable with BSP, High Pressure or Medium Pressure. If you try to thread them together they may seem like they fit at first but will then just damage both fittings and leak as well.
Along with the letters you will often find numbers in the part description as well. It is important to remember that the number usually stands for the first part of a fraction given in 16th’s which indicates thread size. So, for example, an HNM4-NM2 would stand for a 4/16” NPT Male x 2/16” NPT Male. Except no one would say it that way. People will reduce the fraction, just like we did in grade school, and call it 1/4" NPT Male x 1/8” NPT Male when ordering. Other manufacturers will use their own nomenclature like with the HF17 from Hi Force Hydraulic Tools. This fitting is actually a 3/8” Male NPT x 3/8” Male NPT Hex Nipple which will be shown in the description.