What is the difference between access and absorption in Vipassana meditation? I've been studying Visuddhimarga and facing this doubt. In 3.1 (Kammaþþhána-gahaóa-niddesa), regarding how many types of concentration there are, I can get the concepts of distration/non-distraction, bissful/non-blissful, with equanimity/not, etc. However, I don't get what is meant exactly by the terms access and absorption:


  (1) First of all it is of one kind with the characteristic of non-distraction. (2)
  Then it is of two kinds as access and absorption; (3) likewise as mundane and
  supramundane (4) as with happiness and without happiness,


The author gives the following explanation at the bottom of the page, but I'm finding it difficult to understand. I can hazard a guess that absorption here means our thoughts gradually absorb (attain the state of) the object of our concentration, but what is meant by access?:


  
  “Applied thought that occurs as though absorbing (appento) associated states in
  the object is absorption (appaná). Accordingly it is described as ‘absorption, absorbing
  (appaná vyappaná)’ (M III 73). Now since that is the most important, the usage of the
  Commentaries is to call all exalted and unsurpassed jhána states ‘absorption’ [as well
  as the applied thought itself], and likewise to apply the term of common usage ‘access’
  to the limited [i.e. sense-sphere] jhána that heralds the arising of the former, just as the
  term ‘village access,’ etc. is applied to the neighbourhood of a village” (Vism-mhþ 91).
This is more an unfortunate example of problematic translation than anything. appento, for example, means rushing forward, plunging or entering into, or fixing upon. The commentary surrounding this word, 


  sampayuttadhamme ārammaṇe appento viya pavattatīti vitakko appanā


would better be translated as 


  "Applied thought that proceeds as though plunging into the object with its associated states is called appanā"


vyappanā could mean several things, since it is just appanā with a vi prefix. You'd have to be fluent in Pali colloquialism to understand the meaning, but PED says 


  Vyappanā (f.) [vi+appanā] application (of mind), focussing (of attention) Dhs 7. 


At any rate, "absorption" should be understood in the sense of "becoming absorbed in (i.e. fixed upon) the object", I think. 

"Access" is another ambiguous term; it is used in the sense of moving (cara) close to (upa), as it is a translation of the term upacāra. Other translations are "neighbourhoood", "approach", etc. It is more often used in a mundane sense to refer to the area surrounding (e.g.) a dwelling. e.g.:


  vihārassa upacāre - in the surrounding area of a dwelling (Vin. Pāc. 2.5)


In regards the section of the Visuddhimagga you are reading, we should first be clear that this section pertains to samatha meditation, not vipassana. The two terms are not commonly used in the practice of insight meditation, since it is understood that appanā samadhi doesn't allow for insight to occur. Insight meditation requires momentary concentration similar to that described as upacāra in this section, in that it comes close to but doesn't reach samatha jhāna.

The main difference between the two is that appanā is fixed on the object from moment to moment, unwavering and unchanging - it is sometimes described as "trance" concentration; whereas upacāra is still not fixed on a single object. The main similarity is that they both have the power to suppress the five hindrances from the mind.