The User's Guide gives a nice explanation of definition variables and a few examples: http://openmx.psyc.virginia.edu/docs/openmx/latest/DefinitionMeans_Path.html

The short description I would give is a definition variable is a column of your observed data that you use IN YOUR MODEL to define relationships. For example, you might use a variable in your data as the value for a path between two other variables. If the first 5 values of VariableDef are 1, 2, 3.3, 7, 1.4 then you could have a path between X and Y take on the value 1 for the first person, 2 for the second person, 3.3 for the third person, and so on.

Definition variables are often useful for behavioral genetics models where different people have different coefficients of relatedness, and latent growth curves where people were measured at different times.

The User's Guide gives a nice explanation of definition variables and a few examples: http://openmx.psyc.virginia.edu/docs/openmx/latest/DefinitionMeans_Path.html

The short description I would give is a definition variable is a column of your observed data that you use IN YOUR MODEL to define relationships. For example, you might use a variable in your data as the value for a path between two other variables. If the first 5 values of VariableDef are 1, 2, 3.3, 7, 1.4 then you could have a path between X and Y take on the value 1 for the first person, 2 for the second person, 3.3 for the third person, and so on.

Definition variables are often useful for behavioral genetics models where different people have different coefficients of relatedness, and latent growth curves where people were measured at different times.