To capitalize means to record an expenditure on the balance sheet as an asset, to be amortized over the future. The opposite is to expense. For example, research expenditures can be capitalized or expensed. If expensed, they are charged against income when the expenditure occurs. If capitalized, the expenditure is charged against income over a period of time usually related to the life of the products or services created by the research.
(Glossary of Important Financial Accounting Terms from How To Keep Score in Business by Robert Follett)