UU Game Terminology - Action Phase

The Action Phase is the third Phase of a Players turn.

ACTION PHASE
Play a card from your hand or DRAW an additional card from the deck.

During your Action phase, you may do one of the following (unless otherwise stated):

OR DRAW a card

IMPOSSIBLE ACTIONS

If you have a card in your hand that requires you to perform an impossible action, you may not play it. Sometimes, however, the effect of a card in your Stable/Field or a card played by any other player may require you to perform an impossible action. If this situation occurs, disregard that action. Some impossible actions include:

DISCARD when you have no cards in your hand.
SACRIFICE a card when you have no cards in your Stable/Field.
SACRIFICE a card that cannot be sacrificed

Specific Rules and Information

A Phase is not completed until all effects have been resolved, then gameplay continues to the next phase, unless a game winning condition is achieved by the player and the game ends.

Effects may cause phases to be skipped during a turn. It is possible that the same phase can be skipped multiple times due to multiple effects in the turn. (4/3/2019) Reddit @SOmeOneSP3C1AL

The "Action" of playing a card is just the announcement of the intent to play that card. When the card is announced, no effect of the card's text is active (unless otherwise stated). The location of which Stable it will be placed into (upon successful resolution) is NOT required when the card is announced.
A "House-rule" or game variation may be used if all players agree to name the intended Stable on the announcement.
Example: "I am playing Pandamonium, anyone have a response?"
"House-Rule: I am playing Pandamonium into "Bobs" Stable, anyone have a response?"
Both examples are then followed by no other player responding or an Instant Pile is created.(~2018)

If I play a Unicorn card on my turn, can I also play another card? Regardless of card type, you can only play one card during your Action phase, so choose wisely! Some card effects do, however, allow you to bring cards directly into your Stable/Field. You can use as many of these effects during a single turn as your cards allow. (~2018)
This statement does not prevent the playing of Instant cards during the Action Phase, it is only specifically referring to the one card played as the Action. (~2018)

Once a card is played as the Action of the Action Phase and that card is Neigh'd, the one Action for the turn has been completed. The player cannot then choose to play another card. (~2018)

Card effects may allow you to PLAY additional cards during your action phase,
If you choose to DRAW a card as your action you do NOT get to play any card as your action. If you choose to play a card you can also play any additional card from the effect, you do NOT get the option to DRAW a card instead of a card play effect. (~2018)