The letter * i* or

*may refer to any of the following.*

**I**- Usually lower-case —
*i*

- The imaginary unit, defined by , as part of the imaginary number continuum.
- In series, sequences, and other such lists, the index value or variable often found subscripted.
- In vector algebra, (or , or similar) refers to one of the mutually perpendicular coordinate vectors, the former being a unit vector - Along with similar symbols serving similar purpose based on the letters
*j*and*k*.

- Usually upper-case —
*I*

- The identity matrix.
- The unit interval, containing all real numbers from 0 to 1, inclusive.
- In differential equations, the interval of relevance.
- In differential equations, the initial value or initial conditions of a system.
- In infinite series, the interval of convergence.
- In financial mathematics, an interest rate.
- An arbitrary transformation.
- An arbitrary set.

- Either case

- The Roman numeral for the unit number, one (1).
- An arbitrary constant, function name or variable (often integer-valued).

- In various italic or scripted forms

- ℑ - Indicative of the set of
*imaginary numbers*. - ℑ - The imaginary component operation on a complex value.