The story
milli- + nova. From being a transient event akin to a nova, but much less powerful.
Etymology from Wiktionary, CC BY-SA 4.0.
millinova
Today it means: A transient astronomical event, where a bright explosion occurs accompanied by a burst of soft X-rays, becoming a transient soft X-ray source. Explosion source is as yet unidentified, though thought to be produced by white dwarfs, similarly to classical nova, dwarf nova, type Ia supernova, though less powerful. Millinova can be recurrent. Full definition →