Substantially compacted DNA in eukaryotic cells that is less available for transcription.
Heterochromatin can vary in its potential for remaining highly compacted during interphase with some areas constitutively compacted and therefore basically never transcribed versus other areas that are only facultatively compacted, meaning that they on occasion can be subjected to an uncompacting along with subsequent transcription.
See also chromatin and by contrast euchromatin. See too constitutive heterochromatin and facultative heterochromatin.
The basic idea behind heterochromatin is equivalent to how you might store things in your home: The less you need something, then the more compact its storage and the less likely you will need to unpack (this would be 'facultatively' packed away). Of course, somethings are almost never used and these in a sense would remain constitutively packed away.
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