This is the largest diameter of the screw or nut. For instance a ¼” – 20 screw is one having a major diameter of 1/4 inch, and 20 thread per inch.
This is the smallest diameter of the screw or nut. On the nut it corresponds to the tap drill size.
The thread pitch is the distance between threads expressed in millimetres (measured along the length of the fastener). For example, a thread pitch of 1.5 means that the distance between one thread and the next is 1.5mm. In general smaller fasteners have finer thread so they have lower thread pitch.
The thread pitch diameter is the diameter where the thread thickness is equal to the space between the threads. If the flats at the top and bottom of the thread are the same, the pitch diameter will coincide with the middle of the sloping side of the thread.
The root of a tap is the surface at the bottom of the thread-form that connects adjacent thread flanks and is expressed as width or as a diameter.
TPI stands for Threads Per Inch which is a count of the number of threads per inch measured along the length of a fastener. Metric fasteners use a thread Pitch. In general, smaller fasteners have finer threads, so the thread count is higher.