In this rule

Historical Rules · 30 September 2016

Rule 108.2a

Official source text from the 30 September 2016 Rules release

In the text of spells or abilities, the term “card” is usually used to refer to a card that’s not on the battlefield or on the stack, such as a creature card in a player’s hand. The term “card” can also refer to a card in any zone that’s moving “from anywhere.” On rare occasions, the text of a spell or ability may refer to a nontoken permanent as a “card . . . on the battlefield.” For more information, see section 4, “Zones.”

Find this number in the current Rules