What are the OMem and 1Mem columns in the plan output?

The plan output, for example

SELECT *  FROM   TABLE(DBMS_XPLAN.DISPLAY_CURSOR( '' ,NULL,'ALLSTATS  LAST'));

has the columns OMem and 1Mem. What is their meaning? I could not find any reference or documentation about them, just this single web page, claiming (or asking):

OMem:- Estimated optimal mem (PGA) size, to execute the operation in memory.

1Mem:- Estimated size to execute the operation in single pass to disk?