Dirichlet Density is a concept in number theory that helps to understand the distribution of prime numbers in arithmetic progressions. It is defined for a set of integers and measures how many of those integers are prime compared to the total number of integers in that set. This density is particularly useful when studying primes in relation to modular arithmetic. The Dirichlet Density is calculated using the Dirichlet Series, which is a type of infinite series that encodes information about prime numbers. If a set of integers has a positive Dirichlet Density, it indicates that primes are relatively common within that set, while a density of zero suggests that primes are rare.