Some accountancy expert may be able to contradict me but I think these two terms are synonymous and both mean contracts that give rise to liabilities for one of the parties: contracts that produce losses because the costs of complying with them are greater than the benefits they confer.

That is what the Spanish term means, as is clear from the following example:

"Contratos en pérdidas
Con fecha 10 de octubre de 2012, GAMESA suscribió un contrato marco para el suministro de aerogeneradores durante los ejercicios 2013 y 2014. El contrato implica un compromiso de fabricación y venta que GAMESA no puede evitar para aquellas unidades para las que ya se ha recibido y aceptado el pedido. Al 31 de diciembre de 2012, se estimaba que los costes totales superarán los ingresos del contrato, por lo que se registró, de acuerdo con la política contable descrita en la Nota 3.b) de las cuentas anuales consolidadas al 31 de diciembre de 2012, una provisión para riesgos y gastos por importe de 15 millones de euros."
http://www.gamesacorp.com/recursos/doc/accionistas-inversore...

"Definition of 'Onerous Contract'
A type of contract where the costs involved with fulfilling the terms and conditions of the contract are higher than the amount of economic benefit received."
http://www.investopedia.com/terms/o/onerouscontract.asp

"onerous contract
LAW a formal agreement that brings disadvantages for one of the people or companies that have signed it: »
One of the common problems facing businesses is that the business has at some point entered into an onerous contract that adds cashflow burdens without the corresponding benefits."

See also
http://books.google.es/books?id=QQBvtS3oWs0C&pg=PA178&lpg=PA...

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Note added at 8 hrs (2014-07-09 17:53:17 GMT)
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"onerous contract
An agreement that produces a product or service for a larger amount that would be the anticipated profit.
http://www.businessdictionary.com/definition/onerous-contrac...

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Note added at 1 day2 hrs (2014-07-10 11:17:11 GMT)
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This is an accounting term, not a legal term, and in standard international accounting terminology "onerous contract" means this: a loss-making contract.

This is from IAS (International Accounting Standard) 37:
"An onerous contract is a contract in which the unavoidable costs of meeting the obligations under the contract exceed the economic benefits expected to be received under it."
http://ec.europa.eu/internal_market/accounting/docs/consolid...

Accordingly, for example, Deloitte refers to "Onerous (loss-making) contract" under circumstances covered by this IAS.

An onerous contract is one kind of onerous property for insolvency purposes.

Clearly, there can be no legitimate objection to this term as a translation of "contrato en pérdidas". On the contrary, it is the standard term for it, clearly defined as such in International Accounting Standards. I cannot see how it could be misleading in this context.

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Note added at 1 day2 hrs (2014-07-10 11:18:22 GMT)
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Deloitte ref:
http://www.iasplus.com/en/standards/ias/ias37

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Note added at 1 day2 hrs (2014-07-10 11:23:43 GMT)
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On onerous contracts, see IAS 37 sections 66-69 on p. 9 of the source cited:

"Onerous contracts
66. If an entity has a contract that is onerous, the present obligation under the contract shall be recognised and measured as a provision.
67. Many contracts (for example, some routine purchase orders) can be cancelled without paying compensation to the other party, and therefore there is no obligation. Other contracts establish both rights and obligations for each of the contracting parties. Where events make such a contract onerous, the contract falls within the
scope of this Standard and a liability exists which is recognised. Executory contracts that are not onerous fall outside the scope of this Standard.
68. This Standard defines an onerous contract as a contract in which the unavoidable costs of meeting the obligations under the contract exceed the economic benefits expected to be received under it. The unavoidable costs under a contract reflect the least net cost of exiting from the contract, which is the lower of the cost of fulfilling it and any compensation or penalties arising from failure to fulfil it.
69 Before a separate provision for an onerous contract is established, an entity recognises any impairment loss that has occurred on assets dedicated to that contract (see IAS 36)."

Those who are unhappy with this use of "onerous contract" should perhaps take it up with the International Accounting Standards Board at the IFRS, and are of course quite entitled to refrain from using the term in this sense, but I don't think they're entitled to tell accountants to stop using it like this (or at least, it would be rather quixotic to do so).

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Note added at 1 day2 hrs (2014-07-10 11:36:35 GMT)
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There's certainly nothing wrong with "loss-making contract", and if that is preferred it seems fine to me. I have just been concerned here to correct the misapprehension of my esteemed legal colleagues about the term "onerous contract".

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Note added at 1 day3 hrs (2014-07-10 12:16:00 GMT)
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Let me put it this way. If it is argued that "onerous contract" can mean something else so it would be better not to use it in this sense, so as to avoid ambiguity, I can understand that position. I have to say, though, that I find it unlikely that such a problem really exists. If there were any significant risk of misunderstanding, this definition of the term would surely not have been published in the International Accounting Standard and adopted (it would seem) throughout the accounting world. At some point in the drafting process the IAS must have been scrutinised by specialist lawyers, who must have approved it.

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Note added at 1 day4 hrs (2014-07-10 13:28:08 GMT)
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"Onerous contract" is used with the same meaning of a loss-making contract (as defined in IAS 37) in Pricewaterhouse Cooper's UK GAAP 2012 (chapter 21, Provisions):
http://www.bloomsburyprofessionalonline.com/view/manual-acco...

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Note added at 1 day4 hrs (2014-07-10 13:33:30 GMT)
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More:
Also FRS (Financial Reporting Standard) 12:
http://www.accountingweb.co.uk/topic/financial-reporting/gui...

This page gives several examples of onerous contracts. Contracts are not inherently onerous; circumstances make them so. It often applies to leases, where the lease has been abandoned for economic reasons but rent still has to be paid, so the lease is a loss-making contract. But a supply contract can also become onerous, as explained here.

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Note added at 1 day5 hrs (2014-07-10 14:58:45 GMT)
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Bill's right about "unprofitable"; strike that out. Either "onerous" or simply "loss-making".