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"Lying by omission, also known as a continuing misrepresentation or quote mining, occurs when an important fact is left out in order to foster a misconception. Lying by omission includes the failure to correct pre-existing misconceptions. For example, when the seller of a car declares it has been serviced regularly, but does not mention that a fault was reported during the last service, the seller lies by omission. It may be compared to dissimulation. An omission is when a person tells most of the truth, but leaves out a few key facts that therefore, completely obscures the truth."
https://en.wikipedia.org/wiki/Lie
So in that context, withholding information could be construed a lie. But it might get rather blurred in cases where it's debatable whether or not the information was omitted deliberately to cause a false impression. We can't read other people's minds so we can never be sure, and all we can do is suspend judgement or go on what the balance of probabilities seems to be at the time.
Another possible case would be when somebody knows something that they're aware could be helpful to somebody else, when it would be easy and safe for them to divulge. I would say that's not exactly a lie, although it might be irresponsible and selfish, and the "victim" could get very cross about it.
Beyond those special kinds of instances, I wouldn't class omissions as lying at all.