The term intern is not one that has a legal meaning. The law classifies people as either workers or volunteers.
Interns are generally given a list of duties they need to do and fixed times they have to work. This means they’re classified as workers, and legally entitled to the minimum wage. Volunteers, on the other hand, aren’t obligated to do tasks given to them and a volunteer should be able to set hours that are convenient to them. If you’re imposing any of the obligations of a worker onto a volunteer, be mindful that this may entitle them to the minimum wage and the same legal rights as workers. For this reason, using the term ‘intern’ is probably best avoided in relation to volunteers as it can create confusion.