An intercalate is a set of four cells in a latin square such that they can be found in a total of two different columns, two different rows and contain a total of two different symbols.
I’ve marked one of the many examples with red in the picture below.
If the term latin square is unfamiliar, just think of it as kind of a sudoku, but not entirely.
Dictonary.com defines intercalate as
intercalate
[in-tur-kuh-leyt]
verb (used with object), in·ter·ca·lat·ed, in·ter·ca·lat·ing.
1. to interpolate; interpose.
2. to insert (an extra day, month, etc.) in the calendar.Origin of intercalate
1605–15; < Latin intercalātus past participle of intercalāre to insert a day or month into the calendar, equivalent to inter- inter- + calā- (stem of calāre to proclaim) + -tus past participle suffix
Related forms
in·ter·ca·la·tive, adjective
un·in·ter·ca·lat·ed, adjective
Synonyms1. interject, introduce, insinuate.
Intercalat-ed 