I am currently going through some actuarial lecture notes which, without definition, use the term 'central exposed to risk', which it denotes by $E_{x,t}^c$. Having googled this term, the most easily understandable explanation of this term seems to imply that if we have a population of initial size $p$ at time $x$ and at time $x+t$ the size of this population is $q$ then $$ E_{x,t}^c = p - \frac{p-q}{t} $$ i.e. the average number of lives which are alive within the population between times $x$ and $x+t$.
Is this correct?