What does the V stand for in reference to a V8 car engine?

In the terms of an engine V stands for the engine formation. Here the pistons are arranged in two banks such that they appear in the form of the letter V when viewed from the side. In V8, 8 denotes the number of cylinders.

https://en.wikipedia.org/wiki/V_engine

'V engine, or Vee engine is a common configuration for an internal combustion engine. The cylinders and pistons are aligned, in two separate planes or 'banks', so that they appear to be in a "V" when viewed along the axis of the crankshaft. The Vee configuration generally reduces the overall engine length, height and weight compared to an equivalent inline configuration.'

https://en.wikipedia.org/wiki/V_engine

Imagine looking at a "normal" 4-cylinder engine along the crankshaft. The cylinders will be vertical, one behind the other, in the "straight four" or "in-line four" configuration.

https://en.wikipedia.org/wiki/Inline-four_engine

Now imagine separating the tops of the cylinders, 1 and 3 going to the left and 2 and 4 going to the right. Now you have the V4 engine mentioned in the answers above.

Continue the separation until the angle between the cylinders is 180° i.e. the cylinders and the crankshaft between them are all in the same plane. You now have a "flat 4" engine.

https://en.wikipedia.org/wiki/Flat-four_engine