The default shell in macOS Terminal since macOS Catalina is zsh. Apple made Z shell the default login and interactive shell beginning with macOS 10.15 Catalina, released in 2019.
Before Catalina, macOS used Bash as its default shell for many years, beginning with OS X 10.3 Panther in 2003. Apple continued shipping Bash 3.2, but later Bash releases adopted the GPLv3 license, creating licensing concerns for Apple. Zsh offered a powerful alternative with features such as programmable completion, advanced globbing, and flexible prompt customization.
This change affected the shell launched inside Terminal, not Terminal itself. Terminal is a terminal emulator: it provides the window and text interface, while zsh or another shell interprets commands. Users can still select Bash, tcsh, fish, or another installed shell manually. A common mix-up is assuming that Catalina removed Bash; it remained available, although zsh became the default.