Window 1x1 Flat
From Space Engineers Wiki
A standard 1-block pane of glass. It has a different surface on each side, to act as an "inside" and an "outside"; the outside is darker, tinted green and is harder to see through, whilst the inside is tinted blue and is easy to see through.
For ships where the sides are not correct, there is the Window 1x1 Inv, which is identical except that the sides are swapped.