Treas. Reg. § 31.3401(a)-1(a)
(1) The term “wages” means all remuneration for services performed by an employee for his employer unless specifically excepted under section 3401(a) or excepted under section 3402(e).
Obviously, if you are a sole proprietor (or have a single-member LLC), you cannot be both an employee and an employer; you are self-employed and must pay self-employment tax (and generally make estimated income tax payments).
See https://www.irs.gov/businesses/small-businesses-self-employed/self-employed-individuals-tax-center
