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An eTA or an electronic travel authorization is an entry requisite which, foreign nationals from visa-exempt countries should have when travelling to Canada by air. The document does not exist in any physical form and is simply linked to the traveller's passport. The eTA is only exempt for individuals who are from the United States or those who have a valid Canadian visa. As a traveller with an eTA, you will be allowed to visit Canada for five years. This means that there is no unnecessary hassle for you to re-apply for entry and exit.