i0

i0 is not a single universal constant but a label used across disciplines to denote an initial, reference, or incident quantity. The exact meaning depends on context and convention, and in many texts i0 is used interchangeably with I0 (capital i) when referring to intensities or currents.

In electronics and circuit analysis, i0 usually denotes the current at time t=0, the initial current. It appears in solutions to first-order dynamic equations such as i(t) = i0 e^(-t/τ) for a discharging circuit, or as a parameter in transient analyses.

In optics and radiometry, I0 or i0 commonly represents the incident light or radiant intensity on a surface before interaction. It is contrasted with reflected and transmitted components (Ir, It). The exact notation may vary by source; I0 is shorthand for the input irradiance or incident power per unit area depending on conventions.

In acoustics, I0 is the reference sound intensity used to define sound levels, typically I0 = 1×10^-12 W/m^2. In other fields, i0 often marks an initial parameter value in equations, or a baseline orbital inclination i0 in astronomy, underscoring that the interpretation of i0 is context-dependent.