Lightbringer, that would be the bowline knot - one of my favorites! Great for making a secure but temporary loop. It can be pulled apart easily but won't close on itself. Used extensively in rescue situations. So all serious mountain climbers, etc. can ties it one handed as shown in these videos:

https://www.youtube.com/watch?v=H85cMlvwwXA

https://www.youtube.com/watch?v=fJVU0R_YoPE