This term has three meanings: (i) Soil particles of very small size (less than 2µm or 0.002mm). See sand, silt. (ii) Clay minerals, of which there are three main kinds: smectite (an example of which is montmorillonite), illite and kaolinite (iii) A soil which contains enough clay-sized particles or clay minerals to be cohesive. The fraction required varies, but can be as low as 15%. See soil texture.