Variable Length Subnet Masking (VLSM)
A system in which network prefixes have variable length. Typically, classful netmask for an IPv4 address would have a /8, /16, or /24 bit long prefix, depending on the class. In VLSM, a network prefix may be an arbitrary number between 1 and 32, depending on the number of host addresses needed for the subnet.
VLSM, and CIDR (which is based on VLSM) were created to slow the exhaustion of IPv4 addresses by reducing the allocating larger subnets than needed.