Academic rank equivalence between the UK and the US systems
From the wiki pages https://en.wikipedia.org/wiki/Academic_ranks_(United_States) and https://en.wikipedia.org/wiki/Academic_ranks_in_the_United_Kingdom:


Entry level position


UK: Lecturer
USA: Assistant Professor

Mid-level


UK: Senior Lecturer
USA: Associate Professor

Upper-level


UK: Reader
USA: Professor

Highest level


UK: Professor
USA: Endowed Chair/Named Professor



Note 1. that in the USA, ranks #2 and higher are normally tenured. Tenure is handled different in the British system.

Note 2. The American rank of a named chair (aka named professorship, endowed chair) is distinct from the administrative head of a department, who is also often called the "Chair."