Oh, I see. Yeah I suppose it is, now that you point it out. It comes from:
.gov: the US government
.nih: the US National Institutes of Health
.nlm: the National Library of Medicine
.ncbi: the National Center for Biotechnology Information
But really, I only know it because it’s a very common host that comes up when you’re searching for published research papers. I just see “bunch of Ns .gov” and know it’s reliable.