These definitions are usually bundled with standard Windows Updates. If you keep your Windows 10 updated (Settings > Update & Security > Windows Update), Defender will update itself.
Open Command Prompt as an administrator and type:sfc /scannow windows defender windows 10 download 64 bit
If you are searching for a download link for Windows Defender on a standard Windows 10 64-bit machine, These definitions are usually bundled with standard Windows
Remove Third-Party Antivirus: Windows Defender is designed to disable itself automatically if it detects another antivirus program (like McAfee or Norton). If you want to use Defender, uninstalling the other software will usually trigger Defender to turn back on. Key Features of Windows Defender Update & Security >
