is a Cisco NX-OS software image for Nexus switches (64-bit control- and data-plane builds). The naming breaks down as:

In the rack behind him, he heard the faint click of a relay. The active supervisor was still alive. Traffic flowed. No one in the trading floor above knew anything was happening.

: Indicates a 64-bit operating system architecture, which supports larger memory allocation and modern Nexus hardware features.