Many of the calls I get are from caregivers who are panicked over symptoms that are quite normal for someone that is dying. It's okay... I truly understand. This is all strange to us now.
A person that is "actively dying" has made a noticeable transition. They are weakening; sleeping more and eating less (or not eating at all). They need more and more help from caregivers and may be bed-bound. Mental status becomes more "iffy". A fair prognosis would be that the patient has days, maybe even weeks to live.
A patient that is "imminent" is still, of course, considered "actively dying". But their condition has severely deteriorated. They may be semi-conscious/unconscious, confused and hallucinating (or, if you prefer, having "visions"). The ability to swallow is severely impaired. The patient may be anxious, restless or combative. Their breathing pattern has significantly changed. At this stage, one may have hours or days to live.
There are no hard rules for this process. Everyone experiences their death differently... dying is a very personal journey.