A classification of chemical reactions, in which the order is described as first, second, third, or higher, according to the number of molecules (one, two, three, or more) which appear to enter into the reaction; decomposition of H₂O₂ to form water and oxygen is a first-order reaction.

1. A differential equation has order n if the derivatives of a function appear up to the nth derivative.

2. The number of elements contained within a givengroup.

3. A square matrix with n rows and n columns has order n.

4. The number of poles a given elliptic function has in a parallelogram region where it repeats its values.

5. A characteristic of infinitesimals used in their comparison.

6. For a polynomial, the largest exponent appearing in the polynomial.

A range of magnitudes of a quantity (and of all other quantities having the same physical dimensions) extending from some value of the quantity to some small multiple of the quantity (usually 10). Also known as order of magnitude.

A taxonomic category ranked below the class and above the family, made up either of families, subfamilies, or suborders.