Now that is a q uestion. Compost in this instance means a prepared growing medium.There are various recipes for putting them together, but the most generally used ones are those publsihed by the John Innes Institue in California. Each recipe is specifically formulated for a particular use.
No. 1 for seeds and cuttings. No 2 for Growing on and No. 3 for Long term planting Ericaceous for plants which do not like alkaline (lime) soils.
Each one is a mixture in different proportions of sterilised loam, peat, sharp sand, fertiliser and lime.
The Ericacaous ones have more peat than the others and obviously no lime. The fertiliser used in this is also acidic rather than alkaline.
Hope this helps.
In America ask for Acidic compost.

I don't know the trade answere but, for what it is worth, here is my theory.
Most multi-purpose composts are still made from peat which is an acidic material. In order to reduce the acidity lime is added to the mix. As you don't want lime in compost for ericaceous plants you can leave the lime out and call it ericaceous compost. In other words, it is multipurpose compost without the lime.

Grief, my tryping gets worse.

Aha! From the U.S. Forest Service publication, "Growing media alternatives for forest and native plant nurseries" comes this info:

"The first growing medium was called "compost" and was developed in the 1930s at the John Innes Horticultural Institute in Great Britain. It consisted of a loam soil that was amended with peat moss, sand, and fertilizers (Bunt 1988).Soil was heavy and variable, however, so it was difficult to achieve consistency from batch to batch. In the 1950s, researchers at the University of California developed the first true artificial growing media using a series of mixtures of fine sand, peat moss, and fertilizers (Matkin and Chandler 1957). The Cornell "Peat-Lite" mixes, the predecessors of modern growing media, were developed at Cornell University in the 1960s using various combinations of peat moss, vermiculite, and perlite (Mastalerz 1977)."

That explains why the term "ericaceous compost" never came up in my Master Gardener classes. But what is this ericaceous compost without peat that Moon Grower recommended?

The peat-free ericaceous composts that Moon grower talks about use composted pine forest residues such as pine bark and needles, composted heather and composted bracken. This side of the industry is in its infancy still so it does tend to cost rather more than regular peat free products.

Thank you bulbaholic! I haven't seen products quite like that around here so far. I have noticed that many of the potting soils (as we call them in the States) that used to use peat are switching to powdered coir as their main water holding ingredient. I'm still investigating what that means in terms of water retention and pH. One odd note: I talked to a potting soil manufacturer from Oregon about the pH of his products, and he said that the lowest pH soil he produced (still not peat-free) was his Cactus Mix! Go figure. I always used it for camellias, for the sake of drainage, but I used to worry about them getting enough acid. : )

A number of years ago coir was promoted in the UK as a peat substitute but then someone realised that it was being transported many thousands of miles over the sea in ships!. I notice that it is now appearing again so maybe this has been reconciled, although I don't know how.
I never used coir so I don't know its pHn neither am I a cactus grower but I expect that they might be a staple in Arizona, Tugbrethil.

Environmental plus & minus to coir:

+In itself, coir is carbon neutral. Peat adds CO2 to the atmosphere, both by disturbance of the bog, and by decay in it's final destination.
-Coir is often shipped long distances. Most of it comes from Indonesia.
+ & -A modern freight ship is among the most efficient shipping methods known to man. Semi trucks are not, so the carbon benefit is diluted for areas far from the sea.
+Coir is a renewable resource. Peat bogs are running out all over the world, along with the unique ecosystems they sustain.
-Not all coconut plantations are run in an environmentally friendly way.
+On the other hand, many are, and many of the others can be converted easily!
- Shipping unprocessed coir around the world chances introducing exotic pests, diseases, and even weeds to many parts of the globe.

Hmm..pretty much a wash, so far.

What was the question again, 'what is ericaceous compost'? Well peat pure and simple, may be or may not be irish bog peat once sold in bales, the difference is the mix. If there is a mix of anything it ceases to be what it says it is. I bought two bags from my garden centre both labelled ericaceous compost turned it over and there printed on the bag were the words 100% peat.The definition says relating to plants erica which is I think an heather and heathers like acid soil. This although a loose connection is the only one I can make going on the evidence and the fact that a bag has ericaceous printed on it indicates 100 per cent peat..

Thank you, Alladinsane! I guess a medium that is 100% peat would provide the acid needed, but could present problems with drainage, water adsorption, and nutrient balances when used by itself as a potting medium. The more so when used in warmer climates, such as my own!