Disregarded entity
A business with one owner, usually a single-member LLC, that the IRS ignores for income tax. The profit goes on the owner's return.
Your single-member LLC is real in the eyes of the state. For federal income tax, the IRS acts as if it isn't there. That's a disregarded entity. Under Treas. Reg. 301.7701-3, an eligible entity with one owner is, by default, "disregarded as an entity separate from its owner".
What that means for you, if you're an individual owner: the LLC files nothing of its own for income tax. Its profit goes on Schedule C with your Form 1040, and self-employment tax hits it just as it would for a plain sole proprietorship.
There are limits to the disappearing act. Per the IRS LLC page, the LLC still counts as separate for employment taxes and certain excise taxes. Hire someone and the LLC needs an EIN of its own, with payroll run in its name.
Want something else? Form 8832 gets the LLC taxed as a corporation. Form 2553 gets it taxed as an S corp. Either one ends disregarded status, and changing your mind again is hard: the regulation blocks a new classification election for 60 months after the last change.