I think that there is a difference between 'déchéance du terme' of a mortgage and default.

Here is an explanation of 'déchéance du terme' (in French):

Que veut dire "déchéance du terme" ?
Le terme, c'est la date de fin de votre crédit telle qu'elle figure au contrat. Cette date vous permet le plus souvent de rembourser progressivement votre crédit, par amortissement.

Tant que le contrat est respecté, autrement dit si les mensualités sont payées comme prévu, la banque n'a pas le droit d'exiger que vous remboursiez avant le terme.

En revanche, en cas de mensualités impayées, la banque peut, si c'est prévu dans l'acte de prêt, prononcer la déchéance du terme, c'est-à-dire de mettre fin au crédit avant la date prévue au contrat.

Here is an explanation in English of the term 'acceleration' in this context:

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If the borrower defaults and the lender tries to repossess the property, courts of ... Acceleration allows the mortgage holder to declare the entire debt of a defaulted mortgagor due and payable, when a term in the mortgage has been broken.


http://www.law.cornell.edu/wex/acceleration_clause

Acceleration clauses are terms in loan agreements that require the borrower to pay off the loan immediately if certain conditions are met. For example, most home mortgages have an acceleration clause that is triggered if the borrower misses too many payments. Acceleration clauses most often appear in mortgages, both residential and commercial. They also appear in some leases.

From what I can tell it means that if the borrower defaults on the loan, the term of the loan is revoked meaning that the borrower is legally obliged to repay the full amount of the loan and this can be enforced through the courts.
If the borrower is unable to do so, the bank can proceed to repossess the property.