If you are on MacOS, you have probably noticed that MacOS now has zsh as the default shell instead of bash. If you haven't, you are doing a great job of ignoring the motd on opening the terminal :-)
The default interactive shell is now zsh.
To update your account to use zsh, please run
chsh -s /bin/zsh.
For more details, please visit https://support.apple.com/kb/HT208050.
MacOS had bash as the default shell for the Terminal since Panther (OS X 10.3). However, when bash v.4 was released under the GPL v3 license (previous versions of bash were released under the GPL v2 license), Apple decided to switch to zsh. bash is still available on Mojave (OS X 10.14) and Catalina (OS X 10.15) so your bash scripts would still run.
As a developer, the most obvious change is that for changes to the shell environment, you have to edit the .zshrc file instead of the .bashrc file and the .zprofile file instead of .bash_profile. The command history is in .zsh_history instead of .bash_history.