From The Free On-line Dictionary of Computing (19 January 2023):
HOL
Higher Order Logic. A proof-generating system for higher
order logic based on LCF. Implementations include HOL-88
and HOL-90.
(ftp://ted.cs.uidaho.edu/pub/hol). Mailing list:
info-hol@ted.cs.uidaho.edu.
["HOL: A Machine Oriented Formulation of Higher Order Logic",
M.J.C. Gordon, Report 68, Comp Lab U Cambridge (1985)].
["Introduction to HOL", M.J.C. Gordon et al, Cambridge U Press
1993 ISBN 0-521-441897].