義子 is read as ぎし (gishi) and means adopted child.
ぎし
gishi
ぎてい
brother-in-law (spouse's younger brother or younger sister's husband)
ぎけい
brother-in-law (spouse's older brother or older sister's husband)
sister-in-law (spouse's older sister or older brother's wife)
ぎまい
sister-in-law (spouse's younger sister or younger brother's wife)
ぎむ
duty
ぎり
in-law
ぎふ
father-in-law
ぎぼ
foster mother
artificial tooth
ぎむきょういく
compulsory education
ぎだゆう
gidayū (type of reciting used in the puppet theater)
ぎえんきん
donation money (esp. disaster relief or charity)