1999, Brian Brokaw, J. Douglas Arnold, Mark Elies, Pokémon Trading Card Game Player's Guide: Fossil Expansion, Sandwich Islands Publishing, ISBN9781884364396, page 34:
When printed on a card, "damage counter" means the same thing as "10 damage".
2000, Nintendo of America Staff, Paul Shinoda, Official Nintendo Power Pokemon Trading Card Game Player's Guide, illustrated edition, Nintendo of America, Incorporated, ISBN9781930206007, page 75:
Does 20 damage plus 10 more damage for each damage counter on Tauros.
2000, Mike Searle, Consumer Guide, Pokémon Unofficial Card Collector's Guide, illustrated edition, Publications International, ISBN9780785340454:
Remove all damage counters from all of your own Pokémon with damage counters on them, [...]
2000, Hudson Soft, Pokémon Trading Card Game, Nintendo:
[the description of the attack "LEECH SEED" of the card "BULBASAUR LV 13"] UNLESS ALL DAMAGE FROM THIS ATTACK IS PREVENTED, YOU MAY REMOVE 1 DAMAGE COUNTER FROM BULBASAUR.
2000, Jeffrey B. Snyder, Collecting Pokémon: An Unauthorized Handbook and Price Guide, illustrated edition, Schiffer Pub., ISBN9780764310751:
Components of a Pokemon trading card game. A Base Set, Theme Deck, cards, coin, and damage counters.
2003, “Pokemon-e TCG EX Expert Rules”, in Pojo.com, archived from the original on 2003-06-28:
If a Pokémon is Poisoned, put a "Poison marker" on it to show that it's Poisoned. As long as it's still Poisoned, place a damage counter on it after each player's turn, ignoring Weakness and Resistance.
2006, John Kaufeld, Jeremy Smith, Trading Card Games for Dummies, illustrated edition, For Dummies, ISBN9780471754169, page 90:
Calculate how much damage your attack inflicts on the defending pokémon and then put a damage counter on that pokémon for every 10 units of damage it has sustained. (These counters stay on the pokémon until it's knocked out or healed.)
2008, Stephen P. Clouse, “Pokémon TCG Comprehensive Rules”, in (Please provide the book title or journal name), retrieved 2010-11-14:
Damage on a Pokémon is represented by damage counters. Each counter represents 10 damage. Damage is permanent unless it is removed by an effect or modifier or that Pokémon leaves play. A Pokémon can accumulate damage greater than its maximum HP.
2008, Cameron R., “Rules of the Pokémon TCG”, in Legendary Pokémon, archived from the original on 2008-05-25:
Damage counters keep track of the amount of damage a Pokémon has taken. The theme decks sold by Wizards of the Coast contain damage counters. Any small token (such as pennies, stones) will suffice, as long as it is understood that each counter represents 10 damage.