As used in NRS 389.840 to 389.880, inclusive, unless the context requires otherwise:
1. “Basic textbook” or “textbook” means any medium or manual of instruction, including, without limitation, software for computers, containing a presentation of the principles of a subject and used as a basis of instruction.
2. “Supplemental textbook” means any medium or material, including, without limitation, software for computers, used to reinforce or extend a basic program of instruction.
3. A basic or supplemental textbook becomes “unserviceable” when 4 years have elapsed since its removal from the adopted list.