The story
Phono-semantic matching of Japanese ライトノベル (raito noberu), itself a wasei eigo (和製英語; pseudo-anglicism) derived from light + novel.
Etymology from Wiktionary, CC BY-SA 4.0.
light novel
Today it means: A Japanese novel primarily targeting young adults, typically illustrated (in an anime/manga style) and containing not more than 50,000 words. Full definition →