The term pulsar characteristic age (typically symbolized by τ) refers to a particular simplified determination of a pulsar's approximate age, which can be thought of a as the timescale of a pulsar's age. It presumes the pulsar was originally rotating so quickly that its initial rotation period can be approximated by zero, and the variation of its period derivative over its lifetime is following a simple, typical pattern. (The slowing is due to interactions of the pulsar's magnetic field and often follows a particular simple pattern.) The widely-used formula is:
τ = P / ( 2 dP/dt )
Variants to this formula incorporating more data offer a closer, more likely approximation.
Similarly, the pulsar characteristic magnetic field is an approximation of a pulsar's maximum-possible magnetic field:
B = 3.3 × 1019 P dP/dt
The constant 3.3 x 1019 was derived using plausibly common (canonical) values for some pulsar characteristics such as its radius.
A pulsar's braking index indicates a characteristic of the pattern of its spin-down (the slowing of its rotation) over time:
n = Ω d²Ω/dt² / (dΩ/dt)²