ACT

Act refers to a thing that's in existence in the way how it is at the moment in which it is being discussed.  For example, if we're talking about a wood door that is in front of you at this moment, the door in act is the way how it is exactly at this moment.  We're not talking about how it was a second earlier or a second later.

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POTENCY (A.K.A. POTENTIALITY)

Potency and potentiality refer to the same idea and are typically used interchangeably.  Potency refers to a change that a being can undergo.  It can refer to any number of changes including a change in physical position, quality, quantity, substance, color, etc.

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Potency limits a being's act.  For example, an acorn has a potency to become an oak tree and only an oak tree.  This potency limits the act of an acorn because it prevents an acorn from becoming a pine tree or a rock.

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Philosophers differentiate among different type of potency.  The two main categories are passive potency and active potency. 

 

Passive Potency

Passive Potency is typically what philosophers are referring to when speaking about potency.  An object with this type of potency can be acted on by other things to the degree that the potency allows.  For example, water can be a solid (ice).  But if ice is acted on by a flame from a torch, it has the potency to become a liquid.

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Active Potency

Active Potency is often referred to as a Power.  A Power is in act, but it's a special type of act that is in potency to when and how it will be used.  It’s referred to as a active potency because it is not always acting.  A power exists and is in act, but it's in potency to its use.  For example, you have a power within you to move your arm to lift 20 lbs.  Because you currently have this power, this power is in act.  However, because your arm is resting this second while you read this page, this power is in potency to being used.