  Introduced by C. R. Shallon in 1979, graph algebras establish a useful
connection between graph theory and universal algebra. This makes it possible
to investigate graph varieties and graph quasivarieties, i.e., classes of
graphs described by identities or quasi-identities. In this paper, graph
quasivarieties are characterized as classes of graphs closed under directed
unions of isomorphic copies of finite strong pointed subproducts.
