I was using the Chinese (Mandarin) version of WordReference and saw the following label on a button: 回覆
What does this mean? I have been unable to find it any of my dictionaries. I did, however, come across 回复 ("to reply"). Could somebody help me out?
As others said, 回覆 is the traditional form of 回复 (reply).
Simplified and Traditional Chinese characters are not simple one to one mapping.
The 4 characters in SuperXW's post have been merged into 复, meanwhile some meanings of 覆 are kept. In TC there are 4 characters 复 復 覆 複. In SC there are two characters 复 覆 and 覆 only has part of its original meaning.
Some online tools may not do the converting correctly because it's a bit complicated. 回復 is also valid traditional Chinese but the literal meaning is very different ("recover"), and it can be another way to write the standard version 回覆 sometimes. For this specific meaning "reply", 回覆 is the TC and 回复 is the SC.