Astonishing was a pulp magazine that published in the first half of the 20th century. It published many short stories and novels by various authors, although from 1949 forward, its most prominent author was Mark Gordian.
The editor and publisher was Jim McGregor. Aside from being a good read, his most important requirement from a story was that it must be consistent internally, regardless of its implausible premise.
Astonishing is based on Astounding, which later evolved into Analog.