Living Rules Reference · v2.0
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Opponent
During combat, a player’s opponent is the other player that either has ships in the system at the start of the space combat or has ground forces on the planet at the start of a ground combat.
Players who do not have units on either side of a combat are not opponents. Those players cannot use abilities or have abilities used against them that are used against an opponent.
Source: Twilight Imperium: Fourth Edition — Living Rules Reference v2.0 (September 22, 2020), page 25. Rules text reproduced verbatim; the rulebook’s diagrams are omitted.