From a related word or phrase: This is a redirect from a word (or phrase) to a page title that is related in some way. This redirect might be a good search term, or it could be a candidate for a Wiktionary link.
Redirects from related words are not properly redirects from alternative spellings of the same word. They are also different from redirects that are subtopics or related topics, because unlike those, a related word or phrase probably does not warrant its own subtopic section in the target page or possess the possibility of ever becoming an article, template, project page, and so forth.
Look up pulmonary trunk in Wiktionary, the free dictionary.