ATAC = Assay for Transposable-Accessible Chromatin
Chromatin = chromosome DNA plus all proteins involved in its physical structure. DNA in the nucleus is "packaged" into a condensed structure. "Open chromatin" is less densely packaged, making it more accessible to proteins and other molecules (e.g. transcription factors, miRNAs, mutagens), and thus more likely to be transcribed or otherwise used.
ATAC-seq is a method to identify chromatin accessibility (regions of open chromatin).
Tn5 transposase = the enzyme used in ATAC-seq. It binds open chromatin and marks it by inserting known DNA sequences ("adapters").
