1. To mold or carve in relief.
2. To decorate with or as if with a raised design.
When I think of embossing, I tend to envision embossed paper. Obviously, it doesn't end there:
Helmet, Embossed Steel, 1550, Italy
Photo by Mary Harrsch
Philadelphia Museum of Art
Photo by Ben Lepley
Man's Bag, Germany or Holland, 14th century
Los Angeles County Museum of Art
Smoking Set with Masks, 18th century (Edo-Meiji). The netsuke is by Tadatsugu.
Walters Art Museum
Thoroughly Beaten, copper work, Thailand
Photo by Andrew
Leather Neck Corset
by Denby Jorgensen
Photo by postbear
Embossing copper how-to at eHow
How to emboss paper at WikiHow
Paper embossing on Instructables