In common parlance, a fiduciary is someone you can trust. The word itself is related to the Latin word, fidere, meaning to trust. We name our dogs "Fido" because dogs are trustworthy companions. For example, a dog named Hachiko (忠犬ハチ公) faithfully returned every day for nine years to Shibuya Station in Tokyo to greet his master who had passed away.
Corporations certainly place trust in their officers as their agents, but are officers fiduciaries because they are agents? The Restatement (Third) of Agency unequivocally defines agency as a fiduciary relationship:
Agency is the fiduciary relationship that arises when one person (a "principal") manifests assent to another person (an "agent") that the agent shall act on the principal's behalf and subject to the principal's control, and the agent manifests assent or otherwise consents so to act.
An agent is one who represents another, called the principal, in dealings with third persons. Such representation is called agency.