A necropost is a post on an "old, abandoned thread that has been considered 'dead' for a while...that no longer serves any purpose but is bumped back up to the top of the forums by someone posting in it."
If the topic has been inactive for a significant length of time, and the new post adds nothing to the topic, then the post can be considered a necropost. However, a post in a sticky or ITopic is not considered a necropost if it adds more useful information (these topics are kept at the top of the forum page anyway, so posting does not bump them). Necroposting on purpose is strongly discouraged, as it is unhelpful, clogging up the top of the forum with outdated and unneeded topics.
If the topic is not dead, but the post contributes nothing to the discussion, then the post simply counts as spam, which is against the Community Guidelines.
There is no official amount of time that turns a post into a necropost, though when describing a necropost Scratchers may say that bumping a thread after a month or so of inactivity would count as necroposting. However, if the topic is important enough or the post adds more useful information to the topic, then it is not considered a necropost.
The name of this term might be derived from "necromancy", which is a magic that involves reviving someone from the dead. Necroposting is reviving a "dead" post.
Necroposts may happen by accident, where a user posts on a topic without checking the dates of the previous posts.
It is common for Scratchers who are new to the forums to necropost unknowingly because they are unaware of how topics in the forum are bumped when posted on and thus do not realize that necroposts can clutter up the forum pages, or are unaware that doing so is frowned upon in the community.