For corporations and business entities, assets that have a useful life longer than one year and are not held for sale in the ordinary course of business.
The IRS considers almost everything you own and use for personal purposes, pleasure, or investment to be a capital asset. These include items such as stocks and bonds, your primary residence, household furnishings, automobiles used for pleasure or commuting, jewelry, and collections of stamps or coins.
There are specific assets that are specifically excluded from the IRS definition of capital assets (aka noncapital assets). These include property held for sale in the normal course of business, money received or about to be received from the sale of that property, depreciable personal property or real property used for business (such as rental property), protected creative works (such as copyrights on a book), and government publications purchased or received for free from the government.