Mosaic means more than one type of chromosomal makeup, just like mosaic tiles made up of different colors. Down syndrome  mosaic -  trisomy 21 in some cells or cell lines  and some  normal karyotype.  Can occur  in one cell line or across cell lines - skin cells may show, other cell lines mat not. Mosaic Down syndrome can exhibit  the same features and health issues as seen in trisomy 21 or translocation Down syndrome. What determines the severity in presentation is the presence of cells with the normal number of chromosomes. Studies show that the percentage of mosaicism is not an accurate predictor of outcome. Great variability in mosaics, from having very mild  to  major characteristics of  Down syndrome. The percentage of Trisomy 21 cells may vary in different parts of the body. Percentage of trisomic cells in muscle, brain, blood or skin may vary. Thus mosaic trisomy 21 phenotype-karyotype correlations are unpredictable.

Exist a mosaic karyotype when we found two cell lines, this is the case of down syndrome, one cell line with +21 and another normal karyotype.

the phenotype depend the ratio to presence of +21, this is a partial down syndrom.

Mosaic karyotype implies the presence of more than one cell line. In the case of mosaic karyotype of Down syndrome, you have cells with Trisomy 21 and cell with different karyotype that could be normal as well. In practice, geneticists commonly rely on the percentage of cells with trisomy 21 to predict the severity of the phenotype. Of course, that prediction cannot be hundred percent accurate since the percentage of abnormal cells in Leucocytes may be different to that in neurons.

More interesting, you have to be more careful with prenatal testing. The mosaic result Down syndrome could just be a placental mosaic. The test would need to be repeated after birth.