So let first see what is memory address
in PLC there are two type of addressing
Physical input /output addressing
Logical memory addressing in Siemens PLC and binary address in Allen Bradley ( both are same but different name)- this is a logical address which square address and CPU memory card but you are not allowed to use it physically but you can use that address logically. Like in Siemens PLC M0.0,MB40,MW10,MD24 etc.
M= Memory ,
I hope you know,
1 byte = 8 bits
1 word = 2 bytes
1 word= 16 bits
So , memory word is logical address which having 16 bits.