CIDR (Classless Inter-Domain Routing) notation is a compact representation of an IP address and subnet mask, used to express the network and subnet of a device. CIDR notation uses the slash (/) character followed by a number to represent the length of the subnet mask in bits. For example, an IP address of 192.168.1.1 with a subnet mask of 255.255.255.0 can be represented in CIDR notation as 192.168.1.1/24. CIDR notation is commonly used in routing protocols and network management tools to express network addresses and subnets.