lover of a married woman; adulterous lover
Refers specifically to a man who is the lover of a married woman; carries a secretive, illicit nuance. Not a general term for 'boyfriend' or 'lover'.
It seems she has a lover on the side.
Her relationship with her lover was discovered by her husband.
Kanji spelling for 'lover of a married woman'; the characters suggest a secret male lover.
Kana spelling is common in everyday writing, especially when the kanji is obscure or the writer prefers a softer look.
Compound of 密 (secret) and 夫 (husband/man), literally 'secret man'. The exact historical derivation is uncertain, but the kanji clearly convey the meaning of a clandestine male lover.