IPA: /ˈθɪə.rɪst/
KK: /ˈθiərɪst/
A person who develops or proposes ideas and explanations about a particular subject, often in a scientific or academic context.
The theorist presented a new model to explain the behavior of the universe.
The word 'theorist' is formed from 'theoria' (from Greek 'theoria', meaning contemplation or speculation) and '-ist' (meaning one who practices or is concerned with). A theorist is someone who engages in contemplation or speculation about a particular subject.
Think of a person who is deeply 'contemplating' ('theoria') ideas and theories — that's what a theorist does.