Column density is a measure of the amount of intervening matter between an observer and the object being observed.

It is typically measured as the number of hydrogen atoms (HI) per square cm (cm²) projected along a particular line of sight, and is designated N_H. The relationship between total extinction, A(V) and column density in our Galaxy is:

N_H/A(V) ~ (1.8-2.2) x 10²¹ atoms cm^-2 mag^-1

For most extragalactic observations it measures the enviromental density of the object and it’s immediate surrounds. For observations inside our Galaxy column density may be more representative of the projected density along a specific line of sight.

Objects with large amounts of hydrogen are quantified by their column density. For example Lyman alpha absorption lines can be related to the column density of the cloud producing the line. Lyman alpha forest lines have typical HI column densities up to 10^17.2 cm^-2. Clouds with column densities between 10^17.2 and 10^20.3 cm^-2 are called Lyman limit systems. Clouds above 10^20.3 cm^-2 are called Damped Lyman systems or Damped Lyman-alpha systems (DLAs).

An area of the sky with a famously low column density for neutral hydrogen is the Lockman Hole, which has an N_H of 4.5 × 10¹⁹ cm^-2. As such the Lockman Hole is valuable for studying the structure of the interstellar medium, of which neutral hydrogen is a major constituent. The Lockman Hole also acts as a kind of window through the interstellar medium which allows astronomers to see further than they would otherwise at extreme ultraviolet and soft x-ray wavelengths.