When is a certain action or card effect a cost in Yu-Gi-Oh?

Answer 1

A 'cost' is something you must do before activating that card, or effect. Cards were often unclear about what part of the text was a cost - in the absence of specific rulings, assume that if the first sentence of the text is telling you to do discard, pay life, tribute something, then it's a cost. There were so many exceptions, judges had to memorise lists of cards that looked like they had a cost, but did not.

Nowadays cards have had a format update to make it much clearer. The trigger condition is listed first, and ends with a colon. The cost is second, or first if there's no activation condition, and ends with a semicolon. Everything after that is the resolvable effect.

A cost is not 'part of the effect', for anything that specifically triggers from effects, such as Dark World monsters.