Keeping the antivirus definitions updated is the best action to ensure the effectiveness of the antivirus software. Updated definitions allow the software to recognize and protect against the latest threats, which is critical for maintaining system security. Even though activating the software and running scans are important, they rely on updated definitions to be fully effective. Therefore, the regular update of definitions is the best practice for optimal antivirus efficiency. Disabling unused features in Defender Antivirus does not necessarily improve its effectiveness and is less important compared to maintaining updated definitions.