Most economists would define full employment as the amount of employment that exists in an economy when there is no cyclical unemployment. That is, it would be the amount of unemployment that exists if there are no people who are laid off as the result of a recession.
This would not be the same as zero unemployment. There would still be people who are frictionally unemployed (who are looking for their first job or who have quit to find a new job they like better) or structurally unemployed (unemployed because their skills are no longer needed in the economy). Economists say that these kinds of unemployment are "good." Therefore, they say the economy could be at full employment even as these kinds of employment remain in existence.