A crouton is a piece of sautéed or rebaked bread, often cubed and seasoned.
It is used to add texture and flavor to salads, notably the Caesar salad.
Croutons are used as an accompaniment to soups and stews, or eaten as a snack food.
The word crouton is derived from the French croûton, itself derived from croûte, meaning “crust”.
Most people consider croutons to come invariably in the shape of small cubes, but they can actually be of any size and shape, up to a very large slice.
A croûton (crouton) is a diminutive form of a croûte (croute), much like a cigarette is a diminutive form of a cigar.
Many people now use croutons for croute, so the usage has changed. (source)
So can hamsters eat croutons?
Yes they can eat them, they are fine for them.
They are not necessarily of nutritional benefit for them but they are not bad for them all the same and will not harm them if they are eaten by them.
Plain ones are the best for them.