For that paticular tap in you're photo the correct term of phrase would be 'diffuser'..............aerators are fitted to different type taps than in the photo.
Either aerator or diffuser will do the job. unless you want to find out what The Real Truth is in a linguistic or engineering purity exercise.
I doubt that even 1% of the population will be able to explain what the official difference between an aerator and a diffuser is.
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