What 34 letter word was added to the oxford dictionary in 1964?

The 34 letter word added to the Oxford English Dictionary in 1964 is "pneumonoultramicroscopicsilicovolcanoconiosis".

It is a made-up word, coined in 1935 by American physician and inventor, E. W. Fenton. It refers to a lung disease caused by inhaling very fine silica dust.