A "mattifying powder" refers to a cosmetic product that helps reduce shine or oiliness on the skin, giving it a more matte, or non-shiny, appearance. Full definition
Finish with mattifying powder on the T - zone. (elle.com)
For instance, before leaving the house in the morning for my Pilates class, I will bounce out of bed, brush my teeth, splash my face with water, apply a light moisturizer with SPF, and then a sweep of mattifying powder. (xojane.com)
Finish by applying our Mineral Mattifying Powder to absorb excess oil and provide a flawless finish. (beautyexpert.com)