What is the meaning of "trait" in biostatistics? My question is related to the question "When and why do I have to use "trait" for multinomial multilevel models with MCMCglmm in R?" But in contrast to this question, I don't want to know, how trait has to be used in MCMCglmm, but what it means in (bio)statistics. The author of MCMCglmm, Jarrod Hadfield, is biologist. So what do biologists or biostatisticians commonly mean by this word? 
(One example to make clear what I'm looking for: If I would be asking for the meaning of "cluster" in statistical papers about educational research, I would be looking for an explanation like "A cluster is a group, to which multiple observations belong to. For example, many students build a class and this class is then a cluster of students. Data with clusters are often analyzed by multilevel models.")
 A: The paper explaining MCMCglmm indicates the sense in which Hadfield used the term "trait," which isn't quite the same as that recommended by the Wikipedia page. In my experience, I think that in common usage Hadfield's usage is generally accepted.
In biology, a trait is some characteristic of an organism. The distinction in usage is whether the "trait" is the name of the characteristic or the actual value of the characteristic. The Wikipedia page says:

For example, eye color is a character of an organism, while blue, brown and hazel are traits. (Original emphasis).

In the MCMCglmm paper, Hadfield uses names of characteristics (tarsus and back) as examples of "traits" rather than their actual values. In practice, I would also tend to use "trait" for the name of the characteristic rather than the value.
A: Trait is endemic to genetics. The originating study here discusses genetic determinants of successful transition to work from school. The documentation in the TraMine package whence mvad originates is pretty paltry. To add to this problem trait seems to be pretty poorly defined in their work. According to them, trait can be a genotype or a phenotype. This is inclusive of trait as a categorical level referring to the inherited number or types of alleles in a gene that predicts disease, or mutations or SNPs thereof, it can be a concentration of mRNA or protein synthesized by such a gene, or even an overt phenotype in the sense of a person's height or weight. Basically, you will need to consult the documentation (There is none) to understand what it really is. Calling ?mvad should tell you.
