From all I have read, it seems that those who "server at the pleasure of a leader" are paid employees that can be terminated at will and without just cause. The termination cannot be the result of illicit reasons like discrimination. This still allows for collorbration and decenting opinions to be shared respectfully with the leader but when the final decision is made by the leader, the employee has a choice to comply with the leaders's direction or not, and opposing actions my result in termination.
I would welcome other definitions but this is how I understand it.
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The government not accounted for by cabinet departments, independent regulatory commissions, and government corporations. Its administrators are typically appointed by the president and serve at the president's pleasure. NASA is an example
They serve at the pleasure of the President. He can dismiss them at any time. Cabinet members do not have term limits
The Executive Branch, meaning the president.
They serve in four year terms. Currently, a president can only serve 2 terms.
That would be all of them except for the 43 men who DID serve as President.