Marginal distribution

A marginal distribution is the distribution of a selected coordinate or subvector of a random vector, obtained from the joint distribution by integrating or summing over the remaining coordinates.

Marginalization sums or integrates a joint probability distribution over unwanted coordinates to obtain a marginal distribution.

A marginal density is a probability density function for one component of a joint probability distribution. Integrating the joint probability density over the other coordinates gives it; for two components, $f_X(x)=\int f_{X,Y}(x,y)dy$.