A cent is the 100th part of a halftone in the Equal Temperament in a logarithmic scale. Because our ears hear ratios rather then distances on a frequency scale, a logarithmic scale is much more natural than a linear one, allowing us to express intervals as distances.
The definition of a cent is: Given a frequency ratio r, its distance expressed in cents is
As an example, the perfect fifth has a ratio of 3/2, and taking the above formula, this is LC(1.5) = 701.9550. A fifth in the Equal Temperament is \( \left( \sqrt[12]{2} \right) ^7 \) (7 halftones), in cents \( LC\left( \sqrt[12]{2}^7 \right) = 700 \) exactly.