This term describes a linear molecule that has a sequence of units which corresponds to the sequence of units in another linear molecule.
For example, the sequence of amino acids in a polypeptide corresponds to the sequence of nucleotide bases of the mRNA which was translated to make it. In bacteria, the sequence of nucleotide bases in an mRNA molecule is colinear with the sequence of nucleotide bases in a DNA segment which was transcribed to make it (though this is not the case for eukaryotes, which have introns).
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.