The letter i or I may refer to any of the following.

Usually lower-case — i

• The imaginary unit, defined by ${\displaystyle i^2=-1}$, 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, ${\displaystyle \hat\imath}$ (or ${\displaystyle \vec\imath}$, 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.