Capital is sunk when it is irreversibly committed to a project.

Sunk Costs
Money that has already been spent. Sunk costs are important because a company may use, for example, an old piece of equipment to make a new product. In this case, sunk costs are positive because no further investment is required. On the other hand, a sunk cost may be negative; for example, that old piece of equipment may break down after its warranty has expired. This means that the owner will not recover the costs no matter what happens.

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Note added at 18 mins (2013-09-13 06:46:50 GMT)
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Les risques auxquels les parties sont confrontées et les investissements à fonds perdus qui doivent être consentis pour mettre l'accord en œuvre peuvent donc avoir [...]

The risk facing the parties and the sunk investment that must be committed to implement the agreement can thus lead to ...
eur-lex.europa.eu - http://is.gd/VtYwyV

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Note added at 20 mins (2013-09-13 06:48:07 GMT)
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or here:
TUNINVEST - Investimenti in Tunisia - http://is.gd/Yar7Vf

This definition can be found in the European Investment Bank Technical Glossary.