What is run time environment?

Simple and Generic definition

Let's not make it so complicated, in context of software engineering a runtime environment is simply an environment that a piece of software needs to run.

For example you might have Googled PC Requirements for GTA-V and you found the following answer:

Minimum System Requirements:

• OS: Windows 8.1 64 Bit, Windows 8 64 Bit, Windows 7 64 Bit Service Pack 1
• Processor: Intel Core 2 Quad CPU Q6600 @ 2.40GHz (4 CPUs) / AMD Phenom 9850 Quad-Core Processor (4 CPUs) @ 2.5GHz
• Memory: 4GB
• Video Card: NVIDIA 9800 GT 1GB / AMD HD 4870 1GB (DX 10, 10.1, 11)
• Sound Card: 100% DirectX 10 compatible
• HDD Space: 65GB

What are these? These are the environment variables which you need to run GTA-V, another way to say the same thing is:

This is runtime environment for GTA-V.

Take example of Node.js, when you say Node.js is JavaScript runtime environment you simply mean that Node.js provides everything (from software perspective) to run JavaScriptic code on the hardware (Mobiles, Laptops, etc). That everything includes interfaces to the Operating Systems of that particular hardwares.

Note: Both of the below statements are correct:

• Node.js is JavaScript runtime environment (RTE).
• Node.js and OS on which Node.js is installed makes Runtime Environment for JavaScript.

Technical definition

The execution model specifies the behavior of elements of the language. By applying the execution model, one can derive the behavior of a program that was written in terms of that programming language.

Ok I know that console.log("Hi") shows me hi on the console, but how this behaviour of that particular string (console.log) is achieved?
Some or all the underlying steps which were taken between the execution of code and logging of the output constitue runtime environment.