Resources for a project report to a project manager and a functional manager. The organizational structure is:

A matrix organizational structure is explicitly defined by resources having dual accountability - reporting to both a project manager for project work and a functional manager for departmental direction. This structure exists on a spectrum from weak to strong matrix depending on the relative authority granted to the project manager versus the functional manager.

An ad hoc structure is a temporary, informal arrangement assembled for an immediate purpose and lacks the defined dual reporting lines described in the question.

A functional organizational structure has resources reporting exclusively to a functional manager, with no formal project manager authority over resources.

A projectized organizational structure has resources reporting solely to the project manager, with no functional manager involvement in directing their day-to-day work.