I am new to this area so forgive me for asking fundamental questions but...under ERISA, what is the definition of a trustee? Under a self-funded plan, is the trustee always the employer? or the plan administrator? or both? Can a trustee be something or someone else than these two entities?

I would be greatly appreciative if a citation could be provided.

The trustee is appointed as such by the trust document and must accept the position and related responsibilities by signing the trust document which spells them out. It can be a bank, the employer, a group of employees, or another type of paid trustee.

This is probably only part of the answer so hopefully more will add to it.

If the employer is a corporation, you need to make sure that it has the power to act as a trustee under applicable state law. For example, a corporation cannot act as trustee under California law (unless it is a financial institution).