In Norse mythology, Mjöllnir is the hammer of Thor, a major god associated with thunder in Norse mythology. Distinctively shaped, Mjöllnir is depicted in Norse mythology as one of the most fearsome weapons, capable of leveling mountains. Though generally recognized and depicted as a hammer, Mjöllnir is sometimes referred to as an axe or club. In the 13th century Prose Edda, Snorri Sturluson relates that the Svartálfar Sindri and Brokkr made Mjöllnir at the command of Loki.
The Prose Edda gives a summary of Mjöllnir's special qualities in that, with Mjöllnir, Thor:
Mjöllnir simply means "crusher," referring to its pulverizing effect. It is related to words such as the Icelandic verbs mölva ("to crush") and mala ("to grind"), and Swedish noun mjöl ("flour"), all related to English meal, mill and miller. Similar words, all stemming from the Proto-Indo-European root *melə, can be found in almost all European languages, e.g. the Slavic melevo ("grain to be ground") and molot ("hammer"), the Russian Молот (molot - "hammer"), the Greek μύλος (mylos - "mill") and the Latin malleus "hammer", from which English mallet derives, as well as the Latin mola ("mill").
... would be able to strike as firmly as he wanted, whatever his aim, and the hammer would never fail, and if he threw it at something, it would never miss and never fly so far from his hand that it would not find its way back, and when he wanted, it would be so small that it could be carried inside his tunic.
An alternative theory suggests that Mjöllnir might be related to the Russian word молния (molniya) and the Welsh word mellt (both words being translated as "lightning"). This second theory parallels with the idea that Thor, being a god of thunder, therefore might have used lightning as his weapon.