There must exist some french (or other culinary) term for the various ways to describe the different directions to slice an onion.
I'm looking for two terms:
Slicing across the grain, perpendicular to the poles, parallel to the equator, along the latitude.
Slicing with the grain, in the direction of pole-to-pole, along the longitude.
What is the shortest, most concise way to describe these techniques?
I don't think there's an "official" answer, and using obscure French terms is a good way not to be understood, but for reasonably knowledgeable readers, the most concise terms are definitely slicing latitudinally and longitudinally.
("first, assume a spherical onion...")
I call it "slice the onion into rings" for the latitude lines (see Jefromi's answer) and "slice the onion into wedges" for the longitude lines.
There's not a specific name in French for the difference between latitude and longitude slicing. Slicing generally is done longitudinally and referred to as 'ciseler' and if you want it latitudinally, you have to specify. Dicing an onion into small cubes is called 'emincer'. Generally for veg there are also several names for specific cuts like julienne, baton, macédoine,paysanne, jardiniere, tournette... I can't recall all of them off hand.