Found the reference: PANS-OPS (Doc 8168-OPS/611) Vol 1 (Flight Procedures) Part IV - Chapter 2 Obstacle Clearance (page 4-9).
2.1 Holding Area.
The holding area includes the basic holding area and the entry area:
a) the basic holding area at any particular level is the airspace required at that level to encompass a holding pattern based on the allowances for aircraft speed, wind effect, timing errors, holding fix characteristics, etc.;
b)the entry area includes the airspace required to accomodate the specified entry procedures.
(It goes on to define buffer areas, minimum holding levels and the clearance from obstacles afforded by these concepts.....)