Formal, slightly literary. Often seen in titles of philosophical or doctrinal expositions — e.g. 《
佛教要义》 (Essentials of Buddhism), 《
宪法要义》 (Essentials of Constitutional Law). Compare
要点 (yàodiǎn, key points, more colloquial) and
精髓 (jīngsuǐ, the very essence/marrow).