A conjunction meaning 'not only... but also...' used to add information or list additional items or facts.
彼は英語だけでなく中国語も話せる。
He can speak not only English but also Chinese.
この本は面白いだけでなく、勉強にも役立つ。
This book is not only interesting but also useful for studying.
彼女は優しいだけでなく、頭もいい。
She is not only kind but also smart.