fus : This likely stands for "FIPS" (Federal Information Processing Standards) or possibly something specific to the build or type of software (like a special image type), but in Cisco contexts, fus typically denotes a specific type of software image.
The air-ct2500-k9-2-0-0-0-fus.aes is a software image designed for the Cisco 2500 Series Wireless Controllers. The filename can be broken down into several parts: