At least once' is an expression used to indicate that something has happened or been done at least one time. It is often used to emphasize the importance of doing something or to express the idea that something should be done more than once. It can also be used to indicate that something has happened more than once, but not necessarily every time. For example, someone might say 'I have gone to the store at least once this week' to indicate that they have gone to the store at least one time during the week, but possibly more.