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

entity-relationship model
entity-relationship diagram
ERD

    An approach to data modelling
   proposed by P. Chen in 1976.  The model says that you divide
   your database in two logical parts, entities (e.g. "customer",
   "product") and relations ("buys", "pays for").

   One of the first activities in specifying an application is
   defining the entities involved and their relationships,
   e.g. using an entity-relationship diagram to represent a
   model.

   ["The entity-relationship model: toward a unified view of
   data", P.P. Chen, ACM Transactions on Database Systems 1:1 pp
   9-36, 1976].

   (2009-11-21)