## Glossary of Terms

**Dimensionality**: for a model of the form dy/dt=f(y,t), the dimension of the model is the number of**state variables**.**Initial value**(**initial condition**) : the value of some quantity of interest at some specified time, usually time*t*=0. For example, if a city had two million inhabitants at the start of year 2000, we might write*N*(0)=2, where*N*(*t*) is the population size, in millions, at time*t*, measured as years since the start of 2000.**Initial value problem**: A differential equation, together with an initial condition, such as dy/dt=2-3y, y(0)=2.

(Remember: a differential equation by itself does not determine a single solution; we also need an initial condition to pick out a specific solution.)**Parameter**: constants, such as rate constants that appear in a model.**State variable**: a variable that describes the state of the system. For instance, the population size in a population model, the number of susceptibles or the number of infectives in an epidemic model, or the concentrations of the various chemical species involved in a biochemical reaction network. In simple models, the number of state variables required to specify the state of the system equals the**dimensionality**of the model.