An Irminsul was a kind of pillar

Column

A column or pillar in architecture and structural engineering is a vertical structural element that transmits, through compression, the weight of the structure above to other structural elements below. For the purpose of wind or earthquake engineering, columns may be designed to resist lateral forces...

 which is attested as playing an important role in the Germanic paganism

Germanic paganism

Germanic paganism refers to the theology and religious practices of the Germanic peoples of north-western Europe from the Iron Age until their Christianization during the Medieval period...

 of the Saxon people. The oldest chronicle describing an Irminsul refers to it as a tree trunk erected in the open air. The purpose of the Irminsuls and the implications thereof have been the subject of considerable scholarly discourse and speculation for hundreds of years.
A Germanic god Irmin

Irmin

Irmin may be*Old Saxon irmin "strong, whole", maybe also "strong, tall, exalted" , from Proto-Germanic *erminaz, *ermenaz or *ermunaz, in personal names *An alleged Germanic deity in some currents...

, inferred from the name Irminsul and the tribal name Irminones

Irminones

The Irminones, also referred to as Herminones or Hermiones, were a group of early Germanic tribes settling in the Elbe watershed and by the 1st century AD expanding into Bavaria, Swabia and Bohemia...

, is sometimes presumed to have been the national god or demi-god of the Saxons.