doorpost: the upright side-post of a doorway or gate
This word always names one specific part of a doorway: the vertical post that stands on either side of the opening and frames it. The standard lexicons agree on this, and the Hebrew Bible keeps it carefully separate from the parts around it. It is not the door itself (the swinging leaf), not the horizontal beam across the top, and not the threshold underfoot. When a verse lists those parts, the doorpost stands beside them as its own piece.
The one thing that changes from verse to verse is the kind of building, and the senses below sort the occurrences by that setting because that is how the lexicons themselves group them: the side-posts of an ordinary home, the posts at the entrance of a sanctuary or temple, and the posts of a city gate. The core picture is identical in every case, a fixed upright at the edge of an entrance, so this is one meaning seen in three settings, not three meanings.
What makes the word memorable is not its range, which is narrow, but the weight the Bible loads onto this one spot. The doorpost is where the Passover blood was smeared, where a servant's ear was pierced to bind him to a household, and where Israel was told to write God's words. It marks the boundary between inside and outside, and so it becomes the natural place to mark belonging, protection, and instruction. The richness is in what happens at the doorpost, not in the word for it.