Chromatin is a complex of DNA and proteins (mainly histones). These complexes form extended (euchromatin) or condensed (heterochromatin) fibers. Chromatin fiber represents a form of DNA condensation produced by the compactization of nucleosomes. Although DNA is compacted in chromatin it can be transcribed and replicated.
A chromosome is formed of a highly condensed chromatin. As a result, chromosomes can be defined as the highest level of DNA condensation that results in the formation of single-stranded condensed chromatin composed of a single DNA molecule and numerous proteins.
Following DNA replication, a chromosome consists of two identical highly compacted strands attached to each other in the centromere region. These two DNA strands are called chromatids.
As a result, a chromosome is a highly compact form of chromatin, whereas chromosomes may consist of one strand or two identical DNA strands called chromatids.