What is a diamond in baseball?

Answer 1

The "diamond" is really just a square that is rotated 45 degrees. The corners of the diamond are where the bases are. The sides of the diamond are the pathway between each base, where the baserunner runs from one base to the next. The combination of bases and pathways causes it to look like a diamond when seen from above.

Answer 2

The infield (it's shaped like a diamond)

All the answers keep referring to the 4 bases and the shape turned sideways as in the diamond suit of playing cards. But if you look at the total playing field it takes on the shape of a real finished cut diamond, flat on the sides and curved on the top.

From the corner where the foul lines meet at home plate up to the edge where the infield meets the grass shows the shape of a real diamond. In some parks you could extend the size of the image to included the warning track as the curve of the diamond.