Seinen means young adult male, so any manga that’s in a magazine that targets that specific audience.
People forget than seinen, shonen, and shojo are age demographics (makes sense when you find out what the words mean), and not genres. It shows how pointless these labels are when Ghost In the Shell and K-on are both seinen and have absolutely nothing in common.
I personally enjoy when I see people coming up with hard genre definitions for marketing or distribution labels.
