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Column density is a measure of the amount of something along some particular span, defined as its amount within a volume with the length of that span, the volume having a unit cross-sectional area. A common use in astrophysics is the column density of matter along a line-of-sight toward a star. Column mass density is that matter's mass per unit area, quantified by units such as g/cm². Column number density is that of a count of objects constituting matter (e.g., molecules), quantified by units such as per cm². Either of these two may also be used for some specific defined portion of the matter such as particle type or types, e.g., electron column density, for the column number density specifically of free electrons (i.e., not counting other particles).
Column density is equivalent to surface density, each being a measure of the density through one dimension of a volume. The term surface density is generally used for measures through relatively flat volumes whereas column density is generally used for such a measurement through a volume that traverses some significant distance. The term column density is likely to be used for the case of particles along the line-of-sight to star, whereas surface density, of particles over the course of the thickness of a disk.