Overview
The risk neutral pricing framework hypothesizes the following formula for the discounted price process of given asset.
{% D(t)X(t) = \mathbb{E}_{\mathbb{Q}}[D(T)X(T)|\mathcal{F}(t)] %}
The quoted discount rate is equal to 1 divided by the price of some asset chosen to be the numeraire.
{% D(t) = \frac{1}{Y(t)} %}
The choice of numeraire can often simplify the desired calculation.