A method of sharing variables between program units in Fortran 77. A COMMON block is declared by a statement like:
common /vars/ x,y,z

If this statement appears in two subroutines, then they will both use the same storage area for the variables x, y, and z, so these variables will be shared between them. It is valid but confusing for one subroutine to have
common /vars/ x,y,z
while the other has common /vars/ a,b,c
In this case x in the first subroutine will refer to the same variable as a in the second, and so on.

COMMON blocks are deprecated in Fortran 90, where modules are the recommended technique for sharing data between program units.