またね

Meanings

Casual farewell used among friends, family, or close acquaintances. Implies expectation of meeting again soon.

じゃあね
じゃあね is an equally casual farewell among friends, while またね specifically emphasizes seeing the person again.
バイバイN2
バイバイ is a playful, borrowed-English farewell often used with children or close friends, while またね carries a stronger sense of 'see you again soon'.
元げん気きでねN2
またね is a very casual farewell used with close friends expecting a quick reunion, whereas 元気でね often accompanies a longer or uncertain separation.
ではまたN5
またね is even more casual and friendly, typically used among close friends or children, whereas ではまた is a slightly more neutral casual farewell.

さようならpolite 丁寧語N4
さようなら is the more formal, distancing farewell, whereas またね is casual and used with friends or family.

From また (again) + ね (sentence-ending particle for agreement or softening). The combination became a fixed casual farewell.