What is a project constraint

a project constraint is a limitation of a project that blocks the progression or hinder the phase of the project.

Such limitations must be identified as much as possible during the initial planning stage and resolutions must be made before hand to ensure the smooth flow of the project.

A project constrait is like a limitation or risk in a project. For example, a time constrait. It is a limitation of a project because you have a specific time to finish it. Or any other cons in a project.