In the context of career choice, a person's replaceability is the degree to which their role impact would vary should that position be occupied by the next most likely candidate.

80,000 Hours illustrates this idea with an example:

Suppose you become a surgeon and perform 100 life saving operations. Naively it seems like your impact is to save 100 people’s lives. If you hadn’t taken the job, however, someone else likely would have taken it instead. So your true (counterfactual) impact is less than the good you do directly.

This type of consideration is relevant not only to careers, but to donations (“Would someone else have fulfilled charity X’s funding gap if I hadn’t?”) and fundraising (“If I persuade someone to give money, would they have given it anyway?”).

However, it is often unclear to what an extent replaceability applies for a given action, and 80,000 Hours notes that for many career decisions replaceability has only limited relevance.

Further reading

Christiano, Paul. 2013. Replaceability.
An alternative view.

Kuhn, Ben. 2013. Replaceability in altruism.
A discussion regarding charitable donations.

O'Keeffe-O'Donovan, Rossa. 2014.  What does economics tell us about replaceability?
An analysis with regards to careers.

Rieber, Lila. 2015. The bittersweetness of replaceability.
A more personal approach to the arguments, and a discussion of the various ways in which they can be applied.

Todd, Benjamin. 2015. 'Replaceability' isn't as important as you might think (or we've suggested).
80,000 Hours' assessment.