Final answer:
N.P.T. stands for National Pipe Taper, a standard for tapered threads on pipes and fittings to create a tight seal. It is commonly used in plumbing and piping installations.
Step-by-step explanation:
The stamp 'N.P.T.' on the side of a tap stands for National Pipe Taper, which is a standard for threading used to ensure a tight seal when fitting pipes and fittings together. It is widely used in the United States and Canada for various applications that involve plumbing and the installation of piping systems. Taps with N.P.T. threads are designed to taper, creating a seal by the interference fit as the threads are tightened; therefore, the need for additional sealant is minimized but still often used to assure a complete seal.