Crossword Genius

Letter before 49-Down (3)

Ross

I believe the answer is:

psi

(Other definitions for psi that I've seen before include "foreign letter" , "Greek letter; pounds per square inch (abbrev.)" , "Pounds per sq. in. (abbr.)" , "Pounds per square inch (abbr.); Greek letter" , "Greek character" .)

I've seen this clue in The Wall Street Journal.
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