Honorific term used to refer respectfully to someone else's wife. Formal and somewhat old-fashioned; 奥様 is more common in everyday polite speech.
Please give my regards to your wife as well.
Composed of 令 (れい, a prefix expressing respect) and 室 (しつ, 'room' or 'wife'). The character 室 historically referred to the inner chambers and by extension one's wife.