The Microsoft Security Essentials Definition Updates deploys a package containing the latest virus signatures for the latest release of Microsoft Security Essentials. Keeping Microsoft Security Essentials up-to-date ensures that your computer is protected at all times and that the latest viruses and spyware variations will be detected during scanning.
Of course, you could just leave Microsoft Security Essentials to update its definitions on its own and, obviously, it is advisable you do so. But in case your computer is not permanently connected to the Internet or if you haven’t turned it on for a longer period of time, you should consider either forcing an update or manually updating the latest updates. It is the latter case when you need to download and install the Microsoft Security Essentials Definition Updates.
One important thing is to download the correct definitions for your system’s architecture: 32-bit or 64-bit. Once you grab the setup file you need, just double click on it and follow the on-screen instructions to apply the latest definitions to Microsoft Security Essentials.
The installer contains the complete, up-to-date anti-malware definitions for Microsoft Security Essentials, deploying the scanning engine (mpengine.dll), along with other files, such as mpasbase.vdm, mpasdlta.vdm, mpavbase.vdm, mpavdlt.vdm, mpsigstub.exe.
Please note that Microsoft Security Essentials can only be installed on Windows Vista and Windows 7, as on more recent Windows iterations it is superseded by Windows Defender. Therefore, you should point to the Windows Defender Definition Updates if you are running Windows 8 or higher.
Additionally, Microsoft Forefront Client Security ended its life cycle on July 14th 2015. Therefore, no definition updates will be available in the future for Forefront Client Security.