A Touchstone file was originally a proprietary file format for the eponymous frequency-domain linear circuit simulator[1] from EEsof, launched in 1984 and acquired by HP.
The simulator has been called HP/EEsof, then its engine has been successively included in the jOmega and ADS software suites and it is now owned by Keysight.
The Touchstone simulator has long since been superseded,[2] but its file format lives on.
It later became a de facto industry-standard file format not only for circuit simulators but also for measurement equipment (e.g. vector network analyzers, or VNAs), then later still an EIA standard as part of the Input/output Buffer Information Specification (IBIS) project.
More recently on January 26, 2024 the IBIS Open Forum released version 2.1 which clarified the filename extension, introduced more flexible definitions for reference values of the network, and made corrections to several examples.