noun
another dimension; different level; whole other level
Used figuratively to describe something that is on a completely different level of quality, scale, or nature, often in a positive sense.
彼の才能は別次元だ。
His talent is on a whole other level.
このゲームのグラフィックは別次元の美しさだ。
The graphics of this game are of another dimension of beauty.
桁違い also means 'different order of magnitude', but is more numerical and often used for scale or amount, while 別次元 is broader and can refer to quality or nature.
Compound of 別 (another, different) and 次元 (dimension). The figurative use likely developed from the literal meaning of a separate spatial or conceptual dimension.