With Project Management, you must understand that EVERY project has constraints.  Unfortunately, project managers tend to ignore this and it will come back to haunt them time and time again.  Constraints include time, scope, cost, quality, risk, or any other factors that will limit what your options are when managing a project or deliverable. "Triple Constraint" is a term that originally included time, scope, and cost.  Newer definitions include customer satisfaction, risk, and quality.  If you're preparing to take the PMP® exam, include both the original and newer definitions.  Sextuple constraint just doesn't quite role off the tongue. I try to stress to stakeholders every time they try to expand scope that it will directly impact the other constraints.  If you expand the scope, you will either have to expand cost or time.  If you don't expand either of these two constraints, you're going to increase risk and lower quality.

You'll first read about constraints in section 1.3 of the PMBOK®.  PMI will only refer to them as constraints at that time.  Y0u'll find them referenced at other locations within the PMBOK as project constraints.  What you will not find in the PMBOK 4th edition is an actual definition in the glossary.