Mecha are self-propelling machines that are modeled after humans; some can change into multiple, non-humanoid formats as well. They're usually controlled by an internal pilot or by remote, but are sometimes sentient beings that move autonomously. A mecha's size ranges from bulky, wearable armor to a massive, towering contraption and beyond. While mecha have a wide range of applications, such as making manual labor easier, they are most commonly portrayed as heavily-armed war machines.
Action is about conflict. Whether with guns, blades, fists, or mysterious powers, these anime feature characters in combat - either to protect themselves or the things or people they value, or simply as a way of life.
Seinen (青年), translated as "youth", is a demographic aimed at men from late teens to adulthood who understand kanji. Seinen manga rarely include furigana with kanji, and have a wide variety of topics and themes. Seinen is the counterpart of Josei, a demographic aimed at women.