Colossus 245, Stinker

irrational number, 4 letters, I have s?rd
SURD

gosh, that was quick! Thank you - I spent hours trying to find that one.

(I knew that from adolescence, but had I not, I would simply have googled each of the only possible six variants SARD, SERD, SIRD, SORD, SURD, and SYRD for meaning).

I googled it (surd) and got a village in Hungary! Only had a computer for two years and am self-taught, so still find it hard to navigate my way round for answers - but grateful for sites like this one where I can find help on the occasions when I need it most!