A soilless mix is a mixture of components in which it is possible to grow a plant but which does not contain soil in the traditional sense, that is, no garden soil or compost.

Here is a simple example of a Soilless mix:

50% Perlite
50% Vermiculite

Here's another:

Contributed by moonrawk

40% Castings (Worm i presume although the author does not specify)
30% Vermiculite
30% Perlite

Here however is a warning on Soilless mixes which it is important to note.

Contributed by happy

IF you happen to miss a couple of waterings the plant can dry out quick because the mix will not hold much moisture. Soil is much easier and will provide nutrients throughout the grow, lessening the need for ferts.

contributed by glass joe>> Soilless mediums can contain peat moss (like pro-mix)as an ingredient along with worm castings, perlite, etc.