What is the Geass?

The Geass is a supernatural power granted by Code Geass entities called Cs. Lelouch receives his from C.C. as a "Power of Absolute Obedience." When he makes direct eye contact with someone and issues a command, they must obey it — once. The command is irresistible and the person cannot disobey it regardless of their will.

What are the Geass's limitations?

• One use per person: Each person can only be commanded once. Lelouch must use each command perfectly.
• Eye contact required: It won't work on someone whose eyes he can't see.
• No resistance to groups: He can command individuals, not crowds (though he can broadcast to many simultaneously in specific situations).
• It evolves: As the Geass grows, Lelouch eventually loses the ability to turn it off — his eye constantly activates, which causes the Euphemia catastrophe.

What is the Euphemia incident?

During a ceremony intended to create a peaceful Japanese autonomous zone, Lelouch jokingly uses the conditional "What if I ordered you to kill all Japanese?" near Euphemia while his Geass activates uncontrollably. The command takes hold. Euphemia — a genuinely good person who had designed the peaceful zone — massacres Japanese people. Lelouch is forced to kill her to end it. This is the most catastrophic unintended consequence of the Geass in the series.

Why is the "one command per person" limitation so significant?

It makes every use of Geass a high-stakes decision. He can't fix mistakes with a follow-up command. The Suzaku-killing order he gives (which is ultimately what makes Zero Requiem possible — Suzaku, commanded to "survive," becomes capable of killing Zero) is given in crisis and echoes throughout the series.

What does Geass represent thematically?

The appeal and danger of absolute power over others. Lelouch can make anyone do anything — once. This sounds like unlimited power but actually demands perfect strategy, because mistakes are permanent. It's an allegory for any form of coercive authority: you can force compliance, but you cannot control consequences.