asset – ativo
http://www.proz.com/kudoz/portuguese_to_english/economics/20...

security
[finanças] valores mobiliários
http://www.babylon.com/definition/security/Portuguese

Definiçao de ativo
Em contabilidade o termo ATIVO representa os bens e direitos que a empresa tem num determinado momento, resultante de suas transações ou eventos passados da qual futuros benefícios econômicos podem ser obtidos. Exemplos de ativos incluem caixa, estoques, equipamentos e prédios.
pt.wikipedia.org/wiki/Ativos
asset

Definition
Any item of economic value owned by an individual or corporation, specially that which could be converted to cash. Examples are cash, securities, accounts receivable, inventory, office equipment, real estate, a car, and other property. On a balance sheet, assets are equal to the sum of liabilities, common stock, preferred stock, and retained earnings. From an accounting perspective, assets are divided into the following categories: current assets (cash and other liquid items), long-term assets (real estate, plant, equipment), prepaid and deferred assets (expenditures for future costs such as insurance, rent, interest), and intangible assets (trademarks, patents, copyrights, goodwill).
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security
1. An investment instrument, other than an insurance policy or fixed annuity, issued by a corporation, government, or other organization which offers evidence of debt or equity. The official definition, from the Securities Exchange Act of 1934, is: "Any note, stock, treasury stock, bond, debenture, certificate of interest or participation in any profit-sharing agreement or in any oil, gas, or other mineral royalty or lease, any collateral trust certificate, preorganization certificate or subscription, transferable share, investment contract, voting-trust certificate, certificate of deposit, for a security, any put, call, straddle, option, or privilege on any security, certificate of deposit, or group or index of securities (including any interest therein or based on the value thereof), or any put, call, straddle, option, or privilege entered into on a national securities exchange relating to foreign currency, or in general, any instrument commonly known as a 'security'; or any certificate of interest or participation in, temporary or interim certificate for, receipt for, or warrant or right to subscribe to or purchase, any of the foregoing; but shall not include currency or any note, draft, bill of exchange, or banker's acceptance which has a maturity at the time of issuance of not exceeding nine months, exclusive of days of grace, or any renewal thereof the maturity of which is likewise limited."

2. Property which is pledged as collateral for a loan.
Read more: http://www.investorwords.com/4446/security.html#ixzz10qbku8q...

May be tangible or intangible

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Note added at 43 mins (2010-09-28 18:03:10 GMT)
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This term is very common in Portugal, it appears in every single report and accounts.