the opposite side; the reverse (physical or literal back side of something)
コインの表には天皇の顔があり、反面には年号が刻まれている。
The obverse of the coin has the emperor's face, and the reverse has the year engraved.
紙をひっくり返して反面に書き込んでください。
Turn the paper over and write on the reverse side, please.
その建物は表の顔とは反面に、古い歴史が残っている。
That building, opposite to its facade, holds an old history.
on the other hand; while; although — used to present a contrasting or balancing point
彼は仕事は早いが、反面細かいミスが多い。
He works quickly, but on the other hand he makes many minor mistakes.
都会は便利な反面、自然が少ないという欠点がある。
The city is convenient, but on the other hand it has the drawback of little nature.
給料は高いが、反面労働時間が長い仕事だ。
The pay is high, but on the other hand it's a job with long working hours.