Mask
noun
A temporary face is something you put over your face to cover it. It's a mask.
かめん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
ハロウィンの日の夜、仮面ライダーのコスプレをしながら、チョコレートを一箱平らげたビエトを見ました。
On the Halloween night, I saw Viet wearing the Masked Rider costume and eating a whole box of chocolates.
わぁ、びっくりした！何で仮面なんてかぶってるの？
Ah, you startled me! Why are you wearing that mask?
ヴェネツィアの仮面舞踏会に行って白馬の王子様に会うことが私の夢です。
My dream is to go to the masquerade in Venice and meet a knight on a white horse.