بيتا كان أو فوقه أو مصراعا أو ما دونه
Whether a full verse or more, one hemistich or less.
The definition does not restrict taḍmīn to a fixed number of words. Borrowing a whole verse and borrowing a short segment may have very different expressive consequences and levels of recognizability.
The key distinction is between accidental shared wording and the deliberate integration of a known portion of another poem into one's new composition. Literary context helps identify the latter.