def tril_count_from_matrix_dim(matrix_dim: int):
    """Computes the number of lower triangular terms in a square matrix of a given
    dimension `(matrix_dim, matrix_dim)`.

    Args:
        matrix_dim (int): Dimension of square matrix.
    Returns:
        int: Count of lower-triangular terms.
    """
    tril_count = (matrix_dim ** 2 - matrix_dim) // 2 + matrix_dim
    return tril_count