Purpose This book is an introduction to some of the basic concepts of topology, especially of non-Hausdorff topology. I will of course explain what it means (Definition 4.1.12). The important point is that traditional topology textbooks assume the Hausdorff separation condition from the very start, and contain very little information on non-Hausdorff spaces. But the latter are important already in algebraic geometry, and crucial in fields such as domain theory.
Conversely, domain theory (Abramsky and Jung, 1994; Gierz et al., 2003), which arose from logic and computer science, started as an outgrowth of theories of order. Progress in this domain rapidly required a lot of material on (non-Hausdorff) topologies.
After about 40 years ...