In more general terms "runtime" means the userspace facilities above and beyond the standard system APIs (which often translate directly into syscalls into the kernel). It is a set of agreed-upon data structures and functions for operating on them and often includes facilities and patterns for extending the system with your own data structures.

For example the C runtime has a notion of what a "string" is: it's binary layout in memory and the conventions around it (e.g. an array of utf-8 bytes terminated by a null).

A runtime can be very thin or very complex. The dotnet or Java runtimes are massive by comparison. To the point they generally JIT the intermediate language to produce executable code (whether ahead of time or on-the-fly). Go's runtime has its own notion of threading built on top of the system notion of threads.

A self-contained static binary embeds any runtime implementations it needs into its own binary so it is still using runtime facilities but needs no external libraries.

A standalone or "bare" program can mean one that is built using only syscall primitives. Of course that can be taken further: you can build a true baremetal program that is designed to be copied into memory by the bootloader so it runs without a kernel or OS underneath it. This is, after all, what an OS kernel is: just code built such that the bootloader can jump to a fixed (or designated in metadata) address, handing off a pointer to info about the hardware (such as a DeviceTree) in memory and that's it.

In the early PC days BIOS was basically a set of functions built-in to the hardware (or more often flashed onto EEPROM). More or less a minimal sort of runtime + device drivers that knew how to read keyboard input, print characters to the screen, etc.

Almost everything is built on abstractions. In modern systems EFI or equivalents is a form of runtime + device drivers for early boot and the kernel. The kernel forms that for userspace. And a userspace language runtime can be something like a mini-OS for the code it runs. Going the other direction CPUs themselves are much more like a collection of networked PCs than you might expect.