Definition: A paper that belongs to the top 1% of papers in a research field published in a specified year. The 1% is determined by the highly cited threshold calculated for the research field in the specified year.
A paper published in the past two years that received a number of citations in the most recent two-month period that places it in the top 0.1% of papers in the same field.
Definition: An impact factor is a number that measures of the frequency with which the "average article" in a journal has been cited in a particular year or period. Thomson Reuters
You can see the one year and five year impact factors for journals by clicking on the journal title in the search results list.
Additional source for finding impact factors: