V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

CIDR
       Classless Internet Domain Routing [protocol] (RFC 1519)


The Free On-line Dictionary of Computing (30 December 2018):

Classless Inter-Domain Routing
CIDR

    (CIDR) /sid*r/ A technique that summarises a
   block of Internet addresses in a routing table as an
   address in dotted decimal notation followed by a forward
   slash and a two-digit decimal number giving the number of
   leading one bits in the subnet mask.  For example,
   123.123.123.0/24 specifies a subnet mask of
   11111111.11111111.11111111.00000000 (binary), implying the
   block of addresses 123.123.123.0 through 123.123.123.255.

   CIDR is "classless" because it is not limited to the subnet
   masks specified by Internet address classes A, B and C.

   According to RFC 1519, CIDR was implemented to distribute
   Internet address space more efficiently and to provide a
   mechanism for IP route aggregation.  This in turn reduces
   the number of entries in IP routing tables, enabling faster,
   more efficient routing, e.g. using routing protocols such
   as OSPF.  CIDR is supported by BGP4.

   See also RFC 1467, RFC 1518, RFC 1520.

   (2006-01-26)