A man who regularly hits his wife. It is not a neutral description — it is one of the ugliest labels English has for a man, and using it is an accusation. Compare it with the calmer, more official phrase 'domestic abuser' or 'domestic violence offender', which is what news reports and courts tend to use; 'wife-beater' is blunt, angry, tabloid language. Because it is so heavy, people also throw it around as an insult at men they simply dislike, without evidence, so be careful. There is a second, quite different use you should know about: in American slang, a 'wifebeater' is a white sleeveless undershirt (what we would call a 男性背心), named from a nasty stereotype about the kind of man who wears one. That clothing sense is common in casual American speech but many people find it offensive, and 'tank top' or 'sleeveless undershirt' is the safer word. Both senses are usually written with a hyphen or as one word.