Outlaws are assassins, pirates, mercenaries, rogues, and warlocks, and the term was created in Outlaws of Thunder Junction to batch these different creature types mechanically
This is a core tension underlying this entire array of cards and theres no clear answer
Theres a better timeline out there where Richard Garfield invents energy back in 1993, and this resource becomes as universal to the game as tapping or sacrificing cards
Classifications: Cards may have various classification keywords under their name
And finally we have Terra Eternal, which is a very fitting name for this enchantment
After that, you can play a 5/5 that taps for 5, and while it cant pay for generic costs, knocking out colored mana costs will still put you far ahead even if you only use 3-4 mana