From Bhikkhu Bodhi's introduction to the Samyutta Nikaya that I transcribed awhile back.
In MLDB (Middle Length Discourses of the Buddha) I had changed Ven Nanamoli's expermental rendering of sankhara as 'determinations' back to his earlier choice, 'formations'. Aware that this word has its own drawbacks, in preparing this translation I had experimented with several alternatives. Thos most attractive of these were 'constructions', but in the end I felt that this term too often led to obscurity. Hence, like the land-finding crow which always returns to the shop when land is not close by (see Vism 657; Ppn 21:65), I had to fall back on 'formations', which is colourless enough to take on the meaning being imparted by the context. Sometimes I prefixed this with the adjective 'volitional' to bring out the meaning more clearly.
Sankhara is derived from the prefix sam (=con), "together", and the verb karoti, "to make". The noun straddles both sides of the active-passive divide. Thus sankharas are both things which put together, construct, and compound other things, and the things that are put together, constructed, and compounded.
In Samyutta Nikaya (SN) the word occurs in five major doctrinal contexts:
As the second factor in the formula for depenedent origination, sankharas are the kammically active volitions responsible, in conjunction with ignorance and craving, for generating rebirth and sustaining the forward movement of samsara from one life to the next. Sankhara is synonymous with kamma, to which it is etymologically related, both being derived from karoti. These sankharas are distinguished as threefold by their channel of expression, as bodily, verbal, and mental (II 4, 8-10, etc); they are also divided by ethical quality into the meritorious, demeritorious and imperturbable (II 82, 9-13). To convey the relevent sense of sankhara I have rendered the term 'volitional formations." The word might also have been translated "activities", which makes explicit the connection with kamma, but this rendering would sever the connection with sankhara in contexts other than dependent origination, which it seems desirable to preserve.
(2) As the fourth of the five aggregates, sankhara is defined as the six classes of volitions (cha cetanakaya, III 60, 25-28), that is, volition regarding the six types of sense objects. Hence again I render it volitional formations. But the sankharakhanda has a wider compass than the sankhara of dependent origination series, comprising all instances of volition and not only those that are kammically active. In the Abhidhamma Pitaka and the commentaries, the sankharakhanda further serves as an umbrella category for classifying all mental concomittants of consciousness apart from feeling and perception. It thus includes all wholesome, unwholesome, and variable mental factors mentioned but not formally classified among the aggregates in the Sutta Pitaka
(3) In the widest sense, sankhara comprises all conditioned things, everything arisen from a combination of conditions. In this sense all five aggregates, not just the fourth, are sankharas (see III 132, 22-27), as are all external objects and situations (II 191, 11-17). The term here is taken to be of passive derivation - denoting what is conditioned, constructed, compounded - hence I render it simply as 'formations', without the qualifying adjective. The notion of sankhara serves as the cornerstone of a philosophical vision which sees the entire universe as constituted of conditioned phenomena. What is particularly emphasised about sankharas in this sense is their impermanence. Recognition of their impermanence brings insight into the unreliable nature of all mundane felicity and inspires a sense of urgency directed towards liberation from samsara (see 150:20; 22:96)
(4) A triad of sankharas is mentioned in connection with the attainment of the cessation of perception and feeling: the bodily formation, the verbal formation, and the mental formation (IV 293, 7-28). The first is in-and-out breathing (because breath is bound up with the body); the second, thought and examination (because by thinking one formulates the ideas one expresses by speech); the third, perception and feeling (because these things are bound up with mind). Two of these terms - the bodily formation and the mental formation - are also included in the expanded instructions on minfulness of breathing (V 311, 21-22; 312,4-5).
(5) The expression padhanasankhara occurs in the formula for the four iddhipadas, the bases of spiritual power. The text explains it as the four right kinds of striving (V 268, 8-19). I render it 'volitional formations of striving'. Though strictly speaking, the expression signifies energy (viriya) and not volition (cetana), the qualifier shows that these formations occur in an active rather than passive mode.
Apart from these main contexts, the word sankhara occurs in several compounds – ayusankhara (II 266, 19; V 262, 22-23) jivitasankhara (V 152, 29-153,2) bhavasankhara (V 263, 2) – which can be understood as different aspects of the life force.
The past participle connected with sankhara is sankhata, which I translate as ‘conditioned’. Unfortunately I could not render the two Pali words into English in a way that preserves the vital connection between them: ‘formed’ is too specific for sankhata[i], and ‘conditions’ too wide for [i]sankhara (and it also encroaches on the domain of paccaya). If ‘constructions’ had been used for sankhara, sankhata would become ‘constructed’, which preserves the connection, though at the cost of too stilted a translation. Regrettably, owing to the use of different English words for the pair, a critically important dimension of meaning in the suttas is lost to view. In the Pali, we can clearly see the connection: the sankharas, the active constructive forces instigated by volition, create and shape conditioned reality, especially the conditioned factors classified into the five aggregates and the six internal sense bases, and this conditioned reality itself consists of sankharas in the passive sense, called in the commentaries sankhata-sankhara.
Further, it is not only this connection that is lost to view, but also the connection to Nibbana. For Nibbana is the asankhata, the unconditioned, which is called thus precisely because it is neither made up by sankharas nor itself a sankhara in either the active or passive sense. So, when the texts are taken up in the Pali we arrive at a clear picture in fine focus: the active sankharas generated by volition perpetually create passive sankharas, the sankhata dhammas or conditioned phenomena of the five aggregates (and, indirectly, of the objective world); and then, through the practice of the Buddha’s path, the practitioner arrives at the true knowledge of conditioned phenomena, which disables the generation of active sankharas, putting an end to the constructing of conditioned reality and opening the door to the Deathless, the asankhata, the unconditioned, which is Nibbana, the final liberation from impermanence and suffering.