What's it called when someone has loss of vision on the left side of their peripheral vision in their left eye preceded by eye pain/pressure feeling?

Do you mean scotoma? Well, there are a few medical terms that could describe your visual loss phenomenon but in point of fact, your pinpoint exactness of what is being lost from where is unlikely to be precisely what's happening anatomically or physiologically...though that's what's apparent to you. What is known to you are the headache or likely migraine symptoms. Have you seen a good headache specialist. May help.