Real Dictionary

fortieth
ordinal number: The fortieth item in a series is the one that you count as number forty.

fortieth in American English
in the fortieth place, rank, group, etc.
preceded by thirty-nine others in a series; 40th
the one following the thirty-ninth
adjective: next after the thirty-ninth; being the ordinal number for 40
noun: a fortieth part, esp. of one (1⁄40)

fortieth in British English
adjective: being the ordinal number of forty in numbering or counting order, position, time, etc. Often written: 40th
noun: one of 40 approximately equal parts of something