A transitive verb meaning to make something deeper or stronger—used to deepen knowledge, understanding, friendships, etc.
彼女は留学経験で日本語の理解を深めた。
She deepened her understanding of Japanese through studying abroad.
共同作業を通じてチームの信頼関係を深めることができた。
Through collaborative work we were able to deepen the team's trust.
専門書を読むことで知識を深めていく。
You deepen your knowledge by reading specialized books.