Someone who is not a professional in a given field.
A common person.
Layperson, someone who is not an ordained cleric or member of the clergy.
A generally ignorant person.
Lay-sister or lay-brother, person received into a convent of monks, following the vows, but not being member of the order.
A person who is untrained or lacks knowledge of a subject.
A person who is not an expert (in the field that is relevant to the context).
Not expert.