A bastion host is your public presence on the Internet. It’s the system that any outsiders must ordinarily connect with to access a system or a service inside your firewall. By design, a bastion hosts is highly exposed, because its existence is known to the Internet. For this reason, firewall builders and managers need to concentrate security efforts on the bastion. You should pay special attention to the host’s security during initial construction and ongoing operation. You want to use them for any other host that’s security critical.”]
Source: https://www.cs.ait.ac.th/~on/O/oreilly/tcpip/firewall/ch05_01.htm