A “halo” engagement ring is simply a ring that features small diamonds all the way around the center stone, creating a resemblance of a halo. Oftentimes the halo stones will give the appearance that the center stone is much larger. Here is an example of a halo engagement ring:

For more varieties of halo engagement rings, take a look here:

http://www.mervisdiamond.com/catalogsearch/result/?q=halo&x=6&y=14