The Meletians were one of a large group of denominations in early Christian history. The point with which they broke with the larger church was about the ease with which lapsed Christians reentered the church. They were named after Meletius of Lycopolis.
In 325 the Council of Nicaea, at which Constantine was present, attempted to incorporate the Meletians into a unified church. The council agreed to grant Meletian priests "full clerical privileges" if they were willing to forswear schism and "acknowledge the authority of the Alexander of Alexandria."
References
Meletians Wikipedia(Text) CC BY-SA