A professional legally required to act in your best interest, not their own.
A fiduciary is held to the highest standard of care and must put your interests ahead of their own compensation. Fee-only financial advisors registered as investment advisers are held to a fiduciary standard, while some salespeople are only held to a lower "suitability" standard. Asking whether someone is a fiduciary is one of the most important questions before taking financial advice.
A fee-only advisor acting as a fiduciary cannot steer you into a fund just because it pays them a commission.