The fundamental histone-containing structural subunit of eukaryotic chromosomes. In most eukaryotic organisms, nuclear deoxyribonucleic acid (DNA) is complexed with an approximately equal mass of histone protein. It was previously thought that the histones formed some sort of coating on the DNA, but in 1974 it was recognized that the DNA-histone complex was in the form of discrete subunits, now termed nucleosomes.