A compound unit used in measurement of the complexity of a DNA preparation. The rate of annealing of DNA is controlled by the second-order rate of nucleation, expressed as C_o/C=1+k₂C_ot, where C is the concentration of single-stranded DNA at time t, C_o is the original concentration of single-stranded DNA (both expressed as the molarity of nucleotides), and k₂ is the second-order rate constant; often represented as a plot of C/C_o against log(C_ot). C_ot_½ taken from such plots is the value of C_ot when C/C_o=0.5.