De pactis

Ulpianus libro quarto ad edictum.  Huius edicti aequitas naturalis est.  Quid enim tam congruum fidei humanae, quam ea quae inter eos placuerunt servare?  Pactum autem a pactione dicitur --- inde etiam pacis nomen appellatum est --- et est pactio duorum pluriumve in idem pacitum et consensus.  Conventionis verbum generale est ad omnia pertinens, de quibus negotii contrahendi transigendique causa consentiunt qui inter se agunt.  [There is a natural equity in this <Praetorian> edict.  For what so accords with human faith as that which men have decided among themselves to observe?  Moreover the word pact (pactum) is derived from agreement (pactio) (the word peace comes from the same etymology).  And agreement is the agreement and consent of two or more persons about the same thing.  There is a general word "agreement"(conventio) relating to everything agreed upon by those who make some contract (negotio contrahendi) or settlement with each other.]

Digestum 2.14.1