Yes, there is such a thing as "lying by omission".
Lets assume you were dating someone and they told you this Friday they were going out for drinks with a "friend".
Later on you discover it was a woman he had a sexual history with. Would you feel like your man deceived or lied to you?
After all he could claim; "I told you I was going to have drinks with a friend. I never said my friend was not a woman!" Was he honest?
Some people will play this game of (if you don't ask me the "exact question" where I have to give you a definitive answer then it is not lying). Clearly they're being deceptive because they know what you're likely to "assume".