A lie of omission is a lie in which someone deliberately withholds pertinent details about something in order to skew someone else's idea of the truth or engender a misconception. Although a lie of omission is not technically a lie because it contains no false information, it is still referred to as one colloquially because it is deliberately misleading.
Lies of omission can occur over both the short and long term. For example, if a car salesman says that a car is regularly inspected yet neglects to say that a major defect was revealed upon the last inspection, he commits a lie of omission in the short term. If a cashier regularly steals money from the cash register yet never informs his boss because his boss never directly asks him about it, he commits a lie of omission in the long term.