Term for helpful information in a form

I'm writing up a proposal and got stumped on the description for this element:

The informative text (in pink) underneath a field is there to provide a reason for the required data, therefore answering a users uncertainty around why they need to fill in this field.

The information behind the 'i' icon plainly says you must be x years old to register so this isn't duplicate information.

Is there a specific term for this type of element within a form?