Il peut s´agir d´un contrat ecrit qui n´est pas tacitement signé, mais dont les seuls actes (ou conduites) des concernés, implique une acceptation des termes.

un contrat "de fait":
http://www.blogintree.com/ressources-humaines/51-embauche-cd...

Même si vous n’avez pas de contrat écrit, vous avez un contrat de fait puisque vous travaillez bel et bien pour cette entreprise.

http://www.categorynet.com/v2/boards/viewtopic.php?t=1142

La feuille de salaire + une collaboration régulière constituent un contrat de fait (CDI).


http://brunorigolt.blog.lemonde.fr/2009/04/27/dossier-cultur...

car le terme même de “société” impliquait un contrat social entre l’Individu et les Autres, avec un échange réciproque d’obligations, contrat non signé mais contrat de fait

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Note added at 1 hr (2009-05-28 21:52:00 GMT)
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Sens équivalent en anglais:
http://law.anu.edu.au/colin/Lectures/Off_Acc.htm

The mirror model for dealing with negotiations leading to a contract does cause some difficulty if it is applied absolutely precisely. That is, if there is the slightest discrepancy between the offer and the purported acceptance, then a court can come to the conclusion that there is no contract. However, there is a particularly important device or argument which can be used to avoid what might be a very inconvenient result in circumstances where the parties have got on with it and proceeded on the assumption that there is a contract. This is what is called acceptance by conduct. We will see a number of examples of this. Its importance is that, when one or both parties have started to perform the contract, a court can come to the sensible conclusion that there is a contract even though the strict rules of offer and acceptance may not have been satisfied.