Heat is the stuff being manipulated here.
Principles are the tools used to do that.
The specific collection and sequencing of words/thoughts/memory-images used to do that can use word-metaphors from many walks of life besides "spells". We could call them recipes, verses, stanzas, procedures, lyrics, vows, rites, vignettes, stories, tales, invocations, constructs, weavings, programs, scripts, subroutines, etc. Prayers and rituals bring in other meaning from MD or they would be on the list too.
It seems to me that each magic culture/tradition would use a different set of words to describe what it is they are doing to manipulate and use heat. I refer you to Roger Zelazny's Madwand series - in which the protagonist has to learn how to do something similar without the benefit of a teacher. Is he manipulating threads, beams, flows, balls, tones, or what? The answer is -- all of the above, depending on circumstance. Different mindsets (principles) make different tasks easier or harder. His Dilvish the Damned series has sorcerers viewing spells (constructs that can be imbued with energy) on different "planes" wherein the construct appears different from different vantage points (principles?). And demons that live in these different energy planes can be summoned and put to work...
I particularly like the cooking analogy in that an experienced chef will only follow the original recipe exactly a certain number of times before they get bored with it and try something just a little different - with good or bad results. The better the cook, the more often they are successful before something goes wrong and they return to the original recipe.
[Added: A one-time cast could be using up a favor or credit owed to you by some entity outside yourself. Charlie the demon owes you one - but just one.]