serum

Reader's Digest 5月号の記事 'Skin Care for Warmer Weather' からの引用です。
Antioxidants are particularly important during the summer, when stronger UV rays can damage unprotected skin. "Not only can too much sun lead to direct DNA damage, but it can also break down collagen and elastin, due to UV-induced free radicals," explains Dr. Shainhouse. Apply an antioxidant serum, such as vitamin C, in the morning after cleansing your face and before applying sunscreen to help strengthen the skin barrier.

この "serum" は皮膚に塗る物質なので、血清ではないですね。辞書を見ます。
・Merriam-Webster: a usually lightweight cosmetic preparation especially for use on the face: specifically: a typically water-based, often concentrated preparation that lacks lubricating and thickening agents
・Vocabulary.com: Technically, serum is the part of blood that doesn’t clot, or any kind of “watery animal fluid.” But serum as skincare refers to watery fluid made of almost anything designed to make skin more beautiful. No animals involved.
・Cambridge English Dictionary: a substance in beauty products that is designed to improve your hair or skin and make it less dry

スキンケア用の物質・材料ですね。