Algebraic Space in the sense of Serre.

Question

I´m reading the paper of Serre Espaces fibrés algebriqués (Séminaire C.C 1958) and I have a question related to the definition of algebraic space. In the introduction of this paper, Serre says that the definition of an algebraic space is the one given in his Faisceux algebriques coherents, but I can not find a precise definition of this thing. Indeed I would like to know what we understand by an algebraic space in the language of schemes because in the above-mentioned article the theory of schemes was not developed and he worked with varieties instead.