Another unhelpful definition from 87 years ago...
Chromatin is the name for the complex of DNA and histone that forms in eukaryotes. 'Naked' DNA is rarely found in complex cells due to the size of the fully unrolled molecule. For example, a human cell has 4 billion base pairs and the distance between two base pairs is 36 / 12 = 3 Å so 3 times 10-10 multiplied by 4 times 109 is 1.2 meters. Since a cell is only 10 micrometers (1/100,000th the length) it is clearly necessary to 'package' the DNA to fit - analagous to compression of data.
Of course, this is the wrong way round in terms of evolution - clearly genomes have only become as complex because they can be packaged this way. In fact, chromatin probably serves a mostly regulatory role; it comes in two forms euchromatin and heterochromatin. The darker staining heterochromatin is condensed and less available to the transciption machinery. That is, it is archived data that cannot be 'read' until it changes form to euchromatin. In this way, cells can control expression of genes by bringing them into and out of storage. Various techniques (such as methylation) alter the state of a gene; indeed the location along the chromosome can affect a gene's rate of transcription.