Heterochromatin is a term used to describe a densely packed form of chromatin that is typically inactive and transcriptionally silent. There are several synonyms for the word heterochromatin that describe similar biological processes, including condensed chromatin, facultative heterochromatin, constitutive heterochromatin, and repressive chromatin. Each of these terms refers to the same phenomenon of chromatin that is tightly packed and thus inaccessible for transcription, and may be associated with specific DNA sequences or modifications. Understanding the subtleties of these synonyms can be helpful in interpreting studies of gene expression and chromatin architecture in different cell types or developmental stages.