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").
Entity-relationship diagrams can be used 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].