First
First adjective - Coming before all others in importance.
Usage example: there are a number of reasons we can't go, but the first is that we don't have the money
Key is a synonym for first in important topic. In some cases you can use "Key" instead the word "First" as an adjective or a noun, when it comes to topics like vital, supreme, elementary, earliest in order.
Nearby Word: firstly
Key
Key adjective - Coming before all others in importance.
Usage example: maintains that Sir Isaac Newton remains the key figure in physical science
First is a synonym for key in important topic. You can use "First" instead the word "Key" as an adjective or a noun, if it concerns topics such as supreme.
Mutual synonyms
How words are described
Cite this Source
Key and First. (2016). Retrieved 2026, August 15, from https://thesaurus.plus/related/first/key
First & Key. N.p., 2016. Web. 15 Aug. 2026. <https://thesaurus.plus/related/first/key>.
Key or First. 2016. Accessed August 15, 2026. https://thesaurus.plus/related/first/key.
Google Ngram Viewer shows how "first" and "key" have occurred on timeline