The default command-line shell in macOS beginning with Catalina is zsh, also called the Z shell.
macOS Catalina, released in 2019, changed the default login and interactive shell from Bash to zsh. The switch did not remove Bash: it remained available, but Catalina included Apple’s Bash 3.2 because newer Bash releases use the GPLv3 license, while Apple’s distribution choices favored the older version. Users could still select another shell if their workflows required it.
Zsh itself predates Catalina by nearly three decades. Paul Falstad created it around 1990 while he was a Princeton student. Its name came from the login name of teaching assistant Zhong Shao. Zsh combined ideas associated with KornShell and tcsh while adding interactive features such as advanced completion, correction, and customization.
Another common confusion is between the shell and the terminal application. Terminal.app, iTerm2, or another terminal emulator provides the window; zsh is the command interpreter running inside it. Tools such as Oh My Zsh add themes and plug-ins, but they are optional customization layers, not replacements for macOS’s underlying default shell.