Is omission the same as lying?

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A lie as defined by wickepedia as " a type of deception in the form of an untruthful satement ,especially with the intention to maintain a secret or reputation ,protect someone's feelings or avoid punishment". Consequently when you are asked by Dave's wife if he was at the bar drinking with the boys and you said yes ,you were truthful in that regard but if you failed to mention that he was drinking with the hot secretary from his office,was that lying ? Wasnt it your intention to maintain the secret of your friend and maintain his reputaion as a loving and faithful husband and hopefully avoid hurting both parties. Not to mention the terrible consequences for Dave's family (both immediate and extended )if this was found out . This argument now leads us to a moral dilemma, should you have spoken the absolute truth and let the chips fall where they may or should you have omitted this glaring fact and let things continue as is.Daves family is intact ,he is happy ,wife is happy in her ignorance and the hot secretary is happy.So no harm done so far ? right.However,dave continues as before and increases the risk of a future discovery and disruption of his family when you could have "spilled the beans" ,brought the situation out in the open and dealt with it now before it got any worse. Some may think I am not morally responsible once i have told the truth I have satiated my moral compass and omission is another situation .What do you say,are you "Guilty by omission " for the consequences as much as the perpretrator ?