In defining grief, there are many things to consider. One of the main ones is that grief is a process, and most often a lifelong process. This process also includes changes in meaning-making and understanding of the world, and the bonds that those parents or the bereaved have with the people they've lost.
If I was to write a definition of grief that would be most valid to the research we now have, those would be the two things I would include. There would be three, actually: the process, the making sense or the meaning-making, and the continuing bonds with the deceased.