Formatting I suppose would be the commonly used term. Though in itself formatting doesn't necessarily mean that style, or using those conventions, but its an accepted use of the word.
A common value convention is using capitals instead of '_'. Also by prefixing the value names with their types. Example;
image_index
would be
intImageIndex
Not sure if Game Maker specified between numbers though. In this case I would use int because it is an integer. Though I myself am guilty of leaving the type off, since I usually can tell by the wording, or looking at the class. In which case the above example gets changed to simply; imageIndex