Put 'interior archway photo' into Google and see what comes up. If your structure serves the purpose shown in this photo, it's pretty much always called an archway, although it doesn't have to be rounded. It also doesn't have to be so 'grand.' We had one in our small Victorian house where I grew up, and it separated our living room from our dining room. Our house was quite small, single story, only three bedrooms, living room, dining room, kitchen, bathroom and back pantry. Yet the opening (wider than a doorway) was always called the 'archway.'
