Ori is the DNA sequence that signals for the origin of replication, sometimes referred to simply as origin. In E. coli, ori is some 250 nucleotides in length for the chromosomal origin (oriC). The plasmid ori sequences are similar to oriC.
Bacteria have a single origin for replication. Eukaryotes have multiple replicons, each with an ori. The replicons range from 40 kb length, in yeast and Drosophila, to 300 kb in plants.
Mitochondrial DNA in many organisms has two ori sequences. In humans, they are called oriH and oriL for the heavy and light strand of the DNA, each is the origin of replication for single-stranded replication.