(Addressing various posts here)
It's been touched-upon that there are various interpretations of what "Jhana 1" can mean, which contributes to varying answers to the OP question. I would just note here some aspects in terms of the Visudhimagga (Therevadan) interpretation, which has been quite well spelled-out, at least since the 5th century CE, and is widely practiced today, particularly by the Burmese lineages of the Mahasi and Pa Auk Sayadaws.
1) This sort of (absorptive) jhana is practically possible, at least in early stages of developing it, only in meditative postures of sitting, lying (e.g. in the Parinibbana posture), and perhaps the standing posture; but hardly so in the walking posture or with other physical activity, as the sensory attention and motor activities of would break absorption.
The khanika samadhi ("momentary") form of concentration is not restricted in this way. This is the concentration associated with vipassana activity, for instance in the 4-element (Mahadhatu) practice that can be done in walking (as taught by Mahasi and PaAuk). And, as I confirmed in discussion last week (on retreat) with a monk originally ordained by Mahasi, but who more recently has worked a couple of decades with Pa Auk Sayadaw, the 'vipassana jhana' (as he understands it) is essentially a form of khanika samadhi.
2) In absorption, at least in the rupa jhana-s 1-3, the senses are in a sense "repressed", but not necessarily completely absent from awareness. That is to say, stimuli such as light, sound, etc. can be noticed at a relatively primitive level: in terms of the abdhidhamma 'cognitive series' model, awareness 'adverts to' and 'receives' stimuli, but the mind lets go at that, does not proceed with 'investigation' (researching associations) or 'determining' (identifying or recognizing, naming), let alone the further kammic 'impulsion' and 'registration' mental micro-cycles of the cognitive series.
Experientially (in terms of phenomenal description), it's as if there were a kind of protective but partially translucent shield surrounding the central mental fixation and stillness – the stimuli appear outside of this, as if at a distance, but disperse or bounce off, don't penetrate it to divert the mind into attentive activity.
Phenomenologically (analytically), it can be compared to the psychedelic effect (at a relatively strong dosage) where stimuli appear at the 'bare' stage of reception, but the higher-order neural activities are inhibited -- do not engage to 'make sense' of them, or take any resultant action.
Last week in retreat, I experienced this vividly in a couple of sessions with 4th jhana (by Visudhimagga-type definition), when emerging after 30-40 minutes to notice that the unrinary bladder was sending urgent distress signals and the hip/low-back pain was screaming, whereas these were not at all perceived during the absorption. That is to say, 'pleasure/pain' were absent, in this case not even noticed at the outer edges of the absorptive shield.
To invoke a distinction used s/t in medicine or psychology, in jhana-s 1-3 sensory input is 'repressed' or functionally diminished, but in jhana 4 it is 'suppressed,' virtually inhibited.
3) 'Nimitta' is essential in this form of absorption, though it's form can vary widely, i.e. not necessarily the visual light or other commonly described appearances. That's because the mind in fact, by the one-pointedness on a static or at least relatively constant object (as distinct from one-pointedness on a changing situation as in khanika samadhi), forms a 'counterpart' mental image or representation (nimitta means 'sign') of the object. This is what the mind absorbs into – a mental counterpart -- symbol might be a good word -- of the object. That's a common characteristic of absorption across all the various possible kinds of object – the mind essentially absorbs into itself.
4) A complicating factor, in the Theravadan interpretation, is that, at advanced stages of practice, fully absorptive jhana-s can be entered and exited very rapidly, which is used in applying vipassana to the details of the absorptive experience. Absorptive jhana is experienced, and then analysized immediately afterward, while the mental echo of it, so to speak, is still quite vivid. This leads to 'knowledge' (nyana) of the detailed nature of the experience, as in stages of the 'gradual practice' of the complete 'purification' (visudhi) process. (This kind of practice may be what Sariputta's reputed method, as documented in M 111, refers to.)
So, this is not to deny that "jhana 1", by some other definition, is impossible outside sitting meditation. In fact, the rapid-file (advanced) jhana-vipassana practice can probably be done while physically active; but I wouldn't try it, for instance, while driving in a Formula-I race.
My understanding and expression of all this is, no doubt, not perfect, but is, I believe, fairly accurate, and to some extent experientially (pragmatically) verified.
(Edited for lacking spaces and spelling, 20150711 16:30 PDT)
