A copyright doctrine where work created by an employee within the scope of their job is automatically owned by the employer, without needing a separate assignment.
Work for hire is a copyright doctrine with two parts. First, when an employee creates copyrightable work — code, writing, design, marketing materials — within the scope of their employment, that work is automatically owned by the employer. No assignment agreement is needed because employment law creates the ownership automatically.
Second (and importantly), work created by an independent contractor is not automatically work for hire — with a few narrow exceptions specified in copyright law. For contractor-created work to qualify as work for hire, the work must fall into one of nine specific categories (like contributions to a collective work or a commissioned work for use in an audiovisual work), and there must be a written agreement stating that it's work for hire.
In practice, this distinction is why every contractor agreement should include an explicit IP assignment clause. You can't rely on the work-for-hire doctrine for most contractor situations, so you need a written agreement saying the contractor assigns all IP they create for you to the company. Missing this is a very common issue that creates problems during investor due diligence.