The 1Nxxxx numbering system is an American standard (now adopted globally) used to mark semiconductor devices. The “1N” means that it is a single junction semiconductor device (i.e. a diode). “4001” is a number given to the smallest diode in the 400x series (4001, 4002, and so on) – the number indicates the voltage, current and power ratings of the diode. A transistor (which has 2 junctions) would be numbered 2Nxxxx.

1-single junction semiconductor device N-Silicon device 400-its a particular series whose max allowed current is 1A. x- it denotes the voltage rating. 1-50V,2-100V,3-200V… 7-1000V.

SO, it is called 1N4001.