ignis

Ignis, ignis is third declension masculine and an i-stem, but an untidy one. The genitive plural is ignium, and the ablative singular appears as both igni, the older i-stem form, and igne, which is commoner in prose. Both are correct; do not treat igni as a dative when it is standing next to a preposition like in or ex.

It is the plain word for fire: a hearth, a signal beacon, the element itself, and metaphorically the fire of love or anger. Keep it apart from flamma, the visible flame, and incendium, the destructive blaze that burns a city down. The fixed pair is igni ferroque, "with fire and sword", ablative of means. English ignite, ignition, igneous. Romance dropped it for focus, "hearth", which gives French feu, Spanish fuego and Italian fuoco, so ignis lives on only in learned vocabulary.