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.