MarginalDistribution

MarginalDistribution[dist,k] represents a univariate marginal distribution of the kth coordinate from the multivariate distribution dist.

MarginalDistribution[dist,{k1,k2,...}] represents a multivariate marginal distribution of the {k1,k2,...} coordinates.

Examples

dist = MultinormalDistribution[{0, 0}, {{1, 0.5}, {0.5, 1}}];
MarginalDistribution[dist, 1]

Please visit the official Wolfram Language Reference for more details.