In simple terms,
abstraction is looking at similarities and ignoring differences.
Abstraction arises from a recognition of similarities between certain objects, situations, or processes in the real world, and the decision to concentrate on these similarities, and to ignore for the time being the differences. Tony Hoare (British computer scientist)
Dahl, O., Dijkstra, E., & Hoare, C. (1972).
Structured programming. New York: Academic Press., p. 83.