Both 'penta-' and '-gon' are of Greek origin. The 'penta-' means five and '-gon' means a shape. So that, a pentagon is a polygon that has 5 sides, and 5 interior angles.
Note: a pentagram/pentacle is a shape of five-pointed star that has a pentagon inside of it enclosing the pentagon with 5 vertices pointed inward. The polygon that connect 5 vertices pointed outward by enclosing the star is a pentagon as well.
The following is examples of number related prefixes such as penta-:
a. hemi- / semi- / demi- means 1/2, b. mono- / uni- means 1, c. di- / bi- means 2,
d. tri- means 3, e. tetra- / quad- means 4, f. penta- / quint- means 5,
g. hexa- means 6, h. hepta- / sept- means 7, i. oct- means 8,
j. nona- means 9, k. deca- means 10,
e.g. hemidemisemiquaver = (1/2)(1/2)(1/2)(1/8 note) = 1/64 note