(n.) Anything that is very puzzling or difficult to explain.

Calvary cross Chinese puzzle Christogram Gordian knot Greek cross Jerusalem cross Latin cross Maltese cross Russian cross T X ankh avellan cross baffling problem bench mark brain twister cardinal point catch chi chi-rho chief thing christcross climax core cornerstone crisis crisscross critical point cross cross ancre cross botonee cross bourdonee cross fitche cross fleury cross formee cross fourchee cross grignolee cross moline cross of Cleves cross of Lorraine cross patee cross recercelee cross-crosslet crossbones crosslet crossword puzzle crucifix cruciform crux ansata crux capitata crux decussata crux gammata crux immissa crux ordinaria dagger dilemma enigma enigmatic question essence essential essential matter ex exing floorer fork cross fundamental gammadion gist gravamen great point heart high point hitch important thing inverted cross issue jigsaw puzzle joker kernel keystone knot knotty point landmark long cross main point main thing material point matter meat milestone mind-boggler mystery node nodus nonplus nub nut to crack oxymoron papal cross paradox pectoral cross perplexed question perplexity pinch pith pivot pons asinorum poser potent cross problem purport puzzle puzzlement puzzler quandary question question mark real issue rood rub salient point saltire sine qua non sixty-four dollar question snag sticker stumper substance substantive point swastika tau teaser the bottom line the point thrust tough proposition trefled cross turning point vexed question voided cross why