(a) “Gas” means natural gas or casinghead gas. The terms gas, natural gas or casinghead gas when used in this article are interchangeable, and any provisions relating to any one of these shall relate to all gas, natural gas or casinghead gas;

(b) “Lease” means a spaced unit, a separately metered formation within the spaced unit, or each tract within a Corporation Commission approved unitization, or a lease which, for tax reporting purposes, has been assigned a production unit number;

(c) “Oil” means petroleum or other crude or mineral oil; and

(d) “Person” means any natural person, firm, partnership, joint venture, association, limited liability company, corporation, estate, trust, and any other group or combination acting as a unit.