user space
also: userland, user mode
The memory and execution environment where user applications and non-privileged processes run, isolated from the kernel's restricted memory space.
User space refers to the portion of a system's memory and CPU execution context reserved for unprivileged user applications and services. It is logically and physically separated from kernel space, where the operating system kernel executes with full hardware access.
When a user application needs to perform privileged operations—such as reading a file, allocating memory, or sending network packets—it must request the kernel through system calls. The CPU switches from user mode to kernel mode, the kernel performs the operation, and then returns to user mode. This isolation protects the system: if a user application crashes or has a bug, it cannot corrupt kernel memory or crash the entire system.
For example, when you run cat /etc/passwd, the cat process executes in user space. When it needs to read the file, it calls the kernel's open() and read() system calls, which temporarily elevate to kernel space to access the disk, then return data back to the user-space process.