Let's start from the everyday meaning of the term, and then move to the technical contexts. Such sequence is natural, and prevents us from being lost in technical details.
The common example of everyday usage that occurs many times in the suttas is:
Sampajaanamusaavaado (sampajaanamusaa) - conscious lie.
Another common example is from the context of Satipatthana:
“Puna capara.m, bhikkhave, bhikkhu abhikkante pa.tikkante sampajaanakaarii hoti, aalokite vilokite sampajaanakaarii hoti, sami~njite pasaarite sampajaanakaarii hoti, sa"nghaa.tipattaciivaradhaara.ne sampajaanakaarii hoti, asite piite khaayite saayite sampajaanakaarii hoti, uccaarapassaavakamme sampajaanakaarii hoti, gate .thite nisinne sutte jaagarite bhaasite tu.nhiibhaave sampajaanakaarii hoti.
"Furthermore, when going forward & returning, he maintains awareness; when looking toward & looking away... when bending & extending his limbs... when carrying his outer cloak, his upper robe & his bowl... when eating, drinking, chewing, & savoring... when urinating & defecating... when walking, standing, sitting, falling asleep, waking up, talking, & remaining silent, he maintains awareness.
To say it simply,
Sampajañña is the awareness that "now I am doing such and such."
http://www.accesstoinsight.org/lib/thai ... tml#tuccho
The prefix 'sam-', as in the verb 'sampajaanaati', does not seem to have here any special meaning. There's no such form without this prefix.
The Sanskrit equivalent would be sampraj~naa - 'to distinguish, discern, recognize'.
The specific technical applications of this term make for more elaborate interpretations, as in fourfold classification of:
1. satthaka-sampajanna - awareness of purpose
2. sappaya-sampajanna - awareness of what is suitable, fitting
3. gocara-sampajanna - awareness of the object
4. asammoha-sampajanna - awareness of non-delusion
Or in contemplating rise and fall of feelings and recognitions:
“Katha~nca, bhikkhave, bhikkhu sampajaano hoti? Idha, bhikkhave, bhikkhuno viditaa vedanaa uppajjanti, viditaa upa.t.thahanti, viditaa abbhattha.m gacchanti. Viditaa vitakkaa uppajjanti, viditaa upa.t.thahanti, viditaa abbhattha.m gacchanti. Viditaa sa~n~naa uppajjanti, viditaa upa.t.thahanti, viditaa abbhattha.m gacchanti. Eva.m kho, bhikkhave, bhikkhu sampajaano hoti.
"And how is a monk aware? There is the case where feelings are known to the monk as they arise, known as they persist, known as they subside. Thoughts are known to him as they arise, known as they persist, known as they subside. Perceptions are known to him as they arise, known as they persist, known as they subside. This is how a monk is aware."
(Sati sutta, SN 5:180)
There is another similar passage which speaks of samadhi (i.e. increased focus of the mind) leading to remembrance-and-awareness:
And which, friends, is the development of concentration which, developed and made much of, leads to mindfulness and awareness? Here, friends, feelings arise known to a monk, known they persist, known they go to an end. Perceptions arise known, known they persist, known they go to an end. Thoughts arise known, known they persist, known they go to an end. Friends, this is the development of concentration which, developed and made much of, leads to mindfulness and awareness. -- D. 33: iii,223.
Katamá ca ávuso samádhibhávaná bhávitá bahulíkatá satisampajaññáya samvattati? Idhávuso bhikkhuno viditá vedaná uppajjanti, viditá upatthahanti, viditá abbhattham gacchanti; viditá saññá uppajjanti, viditá upatthahanti, viditá abbhattham gacchanti; viditá vitakká uppajjanti, viditá upatthahanti, viditá abbhattham gacchanti. Ayam ávuso samádhibhávaná bhávitá bahulíkatá satisampajaññáya samvattati.
However I think that these technical contexts do not change the meaning of the term to 'comprehension' or 'Constant Thorough Understanding of Impermanence'. It's just the content of this awareness which changes.
The content of awareness (sampajañña) during practice depends on a frame of reference which is kept in mind with remembrance (sati). This frame determines the range of phenomena which the practitioner is aware of (pajaanaati).