PLT EXPRESSED CONCERN REGARDING ATC'S USE OF BASE ENTRY INSTRUCTIONS AND THE VARIOUS INTERPRETATIONS OF SAME.

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THIS RPT IS NOT FOR A SPECIFIC EVENT; BUT FOR A COMMON ATC INSTRUCTION WHICH IS NOT DEFINED AND CAUSES CONFUSION. WITH VFR ACFT IN VMC; A COMMON ATC INSTRUCTION FOR AN INBOUND ACFT IS TO 'ENTER (OR RPT) A 3 MI BASE' OR 'ENTER (OR RPT) A 3 MI FINAL' (OR SOME OTHER SPECIFIED DISTANCE); WITH THE GENERALLY ACCEPTED MEANING BEING TO NOT FLY A PATTERN; BUT TO FLY DIRECTLY TO THE POINT DESCRIBED. THE ISSUE IS WHAT IS THAT POINT? THE MORE TROUBLESOME OF THE TWO IS THE INSTRUCTION FOR A '3 MI BASE' (OR SOME OTHER DISTANCE). THESE TERMS ARE NOT DEFINED IN THE AIM OR THE PLT/CTLR GLOSSARY; SO THE MEANING OF THIS PHRASE IS INTERPED BY EACH PLT. DURING RECENT POLL OF A LCL PLT'S ASSOCIATION; THERE WAS A ROUGHLY EVEN SPLIT AMONG PLTS. ABOUT HALF INTERPED THE INSTRUCTION '3 MI BASE' TO MEAN ENTER A BASE LEG; WITH THE BASE LEG BEING THE STANDARD DISTANCE FROM THE THRESHOLD; MERELY EXTENDED OUTWARD THREE MILES. THE OTHER HALF INTERPRETED THE TERM '3 MI BASE' TO MEAN ENTER A BASE LEG WITH THE TURN TO FINAL 3 MI FROM THE THRESHOLD. I QUESTION THE SECOND INTERP; AS I WOULD EXPECT THE INSTRUCTION FOR THAT TO BE 'ENTER BASE FOR A 3 MI FINAL;' BUT THAT IS IRRELEVANT. AMONG A GROUP OF VERY INTELLIGENT AND PROFICIENT PLTS; THERE WAS A 50/50 SPLIT ON THE MEANING OF A COMMON INSTRUCTION; MEANING ABOUT HALF THE TIME THE PLT ISSUED THAT INSTRUCTION IS NOT WHERE THE CTLR EXPECTS HIM/HER TO BE. THE DIFFERENCE BTWN A 3-MI-LONG STANDARD BASE LEG AND A BASE LEG TO A 3 MI FINAL MEANS AN ACFT IS ABOUT 4 MI FROM THE LOCATION THE CTLR IS EXPECTING. A CTLR EXPECTING A 3-MI-LONG BASE COULD LOSE SEPARATION FOR AN IFR INBOUND; AND COULD EVEN CREATE A COLLISION HAZARD. I STRONGLY SUGGEST A REVISION TO THE PLT/CTLR GLOSSARY AND/OR AIM TO DEFINE THESE COMMON INSTRUCTIONS.