A sniffer is a device or program used to detect and monitor network traffic. It is commonly used by network administrators to identify and troubleshoot network issues, as well as to detect malicious activity. The term 'sniffer' comes from the fact that it 'sniffs' out data packets on the network. The crossword solution for the term 'sniffer' is 'NOSE', which is an appropriate description of the function of a sniffer.