In computer architecture, 18-bit integers, memory addresses, or other data units are those that are at most 18 bits (2.25 octets) wide. Also, 18-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size.
18 binary digits have 262,144 (1000000 octal, 40000 hexadecimal) distinct combinations.
Read more about 18-bit: Example 18-bit Computer Architectures