The definition of showrooming is making a purchase on the web after going to a brick and mortar store to look at the merchandise.
Many people like the convenience and reassurance of checking out an item before doing the actual purpose. This means that the local store become an unknowing free showroom for the online merchant.
This seems to happen a lot with televisions, cameras, and other electronic gear.
An offshoot of showrooming is when someone goes to a smaller store for assistance or to get advice from a clerk, and subsequently goes to a big box store where the price is lower to ultimately make a purchase.