Something that, although detached, stands as part of another thing. Typically referred to in reference to property, where an item such as a garage or dock or backyard falls within the land rights of another owner. This item however is more important to someone who is not the landowner, but may own an adjacent property.

The word is derived from from the word appertains.

In Cohen v Whitcomb, the Supreme Court of Minnesota (1919 142 Minn 20) adopted this definition of an appurtenance:

"That which belongs to something else. Something annexed to another thing more worthy."

Bankapedia's Take:

If you are confused by what exactly an Appurtenance is you are not alone. First off, to understand what an appurtenance is you need to understand what an easement is.  Basically an easement is he right of someone other than the land owner to use a particular piece of  another persons land. Usually easements are used if you need to cross someone else's land in order to exit your property. 

So now that we know what an easement it, understand that an appurtenance is a form of an easement and is often referred to as an appurtenant easement. The difference between an appurtenant easement and a gross easement for example, is that an appurtenant easement is meant to benefit the land. Like a walkway extending beyond ones property into another's. 

Hope that helps- you can always check with one of our mortgage professionals online if you have any questions