What is "irminsul"

An Irminsul ( Old Saxon, probably "great/mighty pillar" or "arising pillar") was a kind of pillar which is attested as playing an important role in the Germanic paganism 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.