A fiduciary is legally and ethically obligated to act in the best interest of their client, ahead of their own interests. This is a meaningfully stricter standard than "suitability," which only requires a recommendation to be reasonably appropriate, not necessarily the best or lowest-cost option available.

Registered Investment Advisers are generally held to a fiduciary standard; not every financial professional is, which is why it is reasonable to ask directly whether an advisor is acting as a fiduciary for a given account.