not only ... (but also); not just; as well as
Used to add information, often followed by ～も or ～さえ. Connects nouns, verbs, and adjectives.
彼は日本語だけでなく英語も話せる。
He can speak not only Japanese but also English.
この店は安いだけでなく、品質も良い。
This store is not only cheap, but the quality is also good.
だけでなく is the neutral, everyday way to say 'not only X' while ばかりか adds a stronger, often surprised or emphatic nuance to the additional element.
だけでなく means 'not only... but also' and is more colloquial, often used for contrast, while に加えて is neutral and simply adds an extra item.
Compound of だけ (only) + でなく (negative continuative of だ). The exact historical derivation is straightforward.