A form is essentially a list of questions. Roughly speaking, each such question is a field. In addition to asking questions, forms may also include instructional videos, images, text, page-breaks etc. Each such instructional or logistical elements is also a field.
More generally, fields are the atomic elements that make up forms. Just as matter is composed of atoms, forms are composed of fields.
In technical circles, fields are sometimes referred to as 'form fields'. But the term is only rarely used.