Parent = the comment that this comment replies to.

Grandparent = the parent of the parent.

Thank you!Neverheard of it referred to that way before but it totally makes sense!

Often abbreviated as "GP".

Why many of us do this: It's often easier to refer to previous posts relatively rather than to cite usernames. For "OP", others will know to simply look (or think) back up to the top (or thereabouts). For "parent", it's usually the comment above the one that is being replied to. So parent to your post is what I'm referencing, but I'm now replying to you (and have taken no notice of the usernames because it's easier to focus on the content).

One also often sees the same line of argument continued by other posters, so while the conversation is consistent with two (or more) sides being explored, the posters change. I really like this about HN - it really is a group conversation most of the time.