Who (and when) first defined interval graphs?
I was able to track the first occurences of interval graphs down to

G. Hajos, Über eine Art von Graphen, Int. Math. Nachr. 11 (1957) page 65

This reference refers to a book of (talk) abstracts. In the abstract of Hajos  interval graphs are defined, without actually calling them interval graphs. Moreover it says, that he gives conditions whether a graph is a interval graph and he discusses how to reconstruct the intervals defining the graph, when the graph fulfills these conditions.