$DNA$ is a long polymer of deoxyribonucleotides.

$\Rightarrow$ The length of $DNA$ is usually defined as number of nucleotides (or a pair of nucleotide referred to as base pairs) present in it.

This also is the characteristic of an organism.

For example, a bacteriophage known as $\phi \times 174$ has $5386$ nucleotides, Bacteriophage lambda has $48502$ base pairs $[\mathrm{bp}]$, Escherichia coli has $4.6 \times 10^{6}$ bp and haploid content of human $DNA$ is $3.3 \times 10^{9}$ bp.