What is 45nm or 16nm or 9nm or nano (nm) meter or micro meter(um) in VLSI?

This is the processes technology based on MOSFET gate length.

Let us consider a simple MOSFET, 

In the above diagram L (length) represents the length of the gate; And W (width) represents the width of the gate. The 180nm or 90nm or 45nm or 16nm or 9nm represents this length. When we say the IC is designed with this technology in the sense actually we are representing the gate length of the transistor used in this IC. All the remaining transistors will also have the same gate length. this also described as the smallest pattern which can be drawn on the surface of the silicon chip.

NOTE: Some of may think that 45nm is the actual IC size but it is not correct it represents the transistor gate length. Even it will not represent the entire transistor size.