Coloquially, crescendo is often used--inaccurately--to refer to this. Climax might be used, but a musical climax is not necessarily about volume, and this term is not included in the Oxford Dictionary of Music. I am looking for a word used by composers and/or musicologists to particularly refer to the loudest point in a musical composition.
Unfortunately "rise to a crescendo" has now entered the English language. But I think musicians still know that a crescendo is the climb, not the summit!
If you find "climax" too vague and you specifically want to refer to the LOUDEST point of a piece, I think you'll have to say "loudest point". Or, if you're thinking in terms of compression, normalisation etc. the 'peak level'.
I can't really see that it necessarily has anything to do with volume or intensity, but solely the intention of the composer. The word 'climax' suggests an end, the word 'crescendo' describes the climb - perhaps we start a new thing here and start calling it 'parte migliore' - literally : the best bit!