Definition
A motion is the specific statement a round of debate is organized around, most commonly in parliamentary and British Parliamentary style debate, typically phrased as "This House believes that..." or "This House would...". The motion sets the boundaries of the debate: proposing teams argue for it, opposing teams argue against it, and the wording of the motion is treated as binding on what arguments are legitimately in bounds.
Example
"This House would ban targeted political advertising on social media." A proposing team must argue for this specific policy, not a broader claim like "social media is harmful," and an opposing team's strongest response usually engages the actual mechanism named in the motion rather than social media in general.
Common mistakes
Debaters sometimes drift from the motion as actually worded into a nearby, easier version of the debate. A team assigned to defend banning an activity, but that instead spends the round arguing the activity is merely "not ideal," is not defending the motion as stated. Judges are trained to check arguments against the motion's specific wording, so staying anchored to it matters more than debaters under pressure sometimes realize.