Uniform refers to an item or a group that that is of equal standard, consistent appearance, size or other property over time or area. A plane surface may be uniform if it has no variation or blemishes.
In certain occupations, mainly those that provide a service (e.g. military, police, guards, guides, shop or restaurant workers), then a uniform refers to a specific form of dress or clothing, often adorned with badges or insignia that distinguish that group.