Shonen, literally "boy" in Japanese, is the demographic category for manga and anime made for young male readers, roughly ages 12 to 18. It names a shelf, not a genre: a series is shonen because of the magazine it runs in, whatever its plot is about.
Demographic labels in Japan attach to publications, not stories. A manga is shonen because it is serialized in a shonen magazine, and an anime is shonen because it adapts one or courts the same audience. Weekly Shonen Jump, Weekly Shonen Magazine, and Weekly Shonen Sunday are the flagship magazines, and Jump in particular has an editorial tradition summed up in its three pillars: friendship, effort, and victory.
That shelf happens to hold the biggest adventure franchises in the medium, which is why shonen is often mistaken for a genre meaning battles and power-ups. The shelf also holds sports series, comedies, romances, detective stories, and cooking competitions. What the label predicts is the target reader and the pacing conventions built for them, not the subject matter.
The actual audience is far wider than the target. Shonen series are read by every age and gender, and globally they function as the default shape of mainstream anime. When a fan says "a shonen," they usually mean a long-running action adventure, but the precise meaning is the demographic shelf the work sits on.
In use"A cooking manga in Weekly Shonen Jump is shonen, because the magazine decides the shelf."