A working adult; someone who is a full-fledged member of society, typically employed after finishing school.
来年からは社会人として働き始める。
From next year I'll start working as a member of society (i.e., begin my career).
社会人としてのマナーを身につけることが大切だ。
It's important to learn manners expected of a working adult.
彼はもう社会人だから、時間を守るべきだ。
He's already a working adult, so he should be punctual.