What does port mean in context of a shell in unix?

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http://unix.stackexchange.com – From the book Advanced Programming in the Unix environment: Linux uses the Bourne-again shell for its default shell. In fact,/bin/sh is a link to /bin/bash.The default user shell in FreeBSD and Mac OS X is the TENEX C shell, but they use the Bourne shell for their administrative shell scripts because the C shell’s programming language is notoriously difficult to use.Solaris, having its heritage in both BSD and System V, provides all the shells shown in Figure1.2. Free ports of most of the shells area available on the Internet. What does the author mean by the term port her (HowTos)