The adjective essential (also essential), derived from the Latin essentia, means essential, indispensable, vital. In terms of nutrition, essential substances are those that are needed by the body but cannot be produced by the body itself, i.e. they must be supplied with food. Semi-essential(semi, Latin for half) are substances that are only essential for the body in certain situations (e.g. illness, growth). An example of this are essential amino acids.