The traditional name of the first process started by the Linux kernel is init.
During boot, the kernel initializes enough of the system to launch the first user-space process. Traditionally, that process was called init and was assigned process ID 1. It starts essential services, runs startup scripts, establishes system states, and becomes the ancestor of nearly every other process on the machine.
The name init is short for initialization. If the kernel cannot start the process or if the process with PID 1 terminates unexpectedly, the system normally experiences a kernel panic because no process remains to manage the user-space environment. Init also adopts orphaned processes, helping keep the process hierarchy coherent.
Modern Linux distributions often use systemd as the actual PID 1 process. systemd is a broader service manager and initialization system that replaces traditional System V-style init on many distributions. That does not make “systemd” the traditional name: the historical and generic answer to this question remains init. Bash is a shell, and the kernel itself is the component that launches init, not the process being launched.