Long Grain

Long grain refers to a surface or a glue joint that runs parallel to the direction of the wood fibers, as opposed to end grain, where the fibers are cut crosswise and exposed as open tube ends. Long-grain-to-long-grain glue joints -- edge-gluing two boards into a panel, for instance -- are exceptionally strong, often stronger than the wood itself, because the glue bonds to intact fiber walls rather than soaking into hollow fiber ends.

It's the reason edge gluing a panel from narrower boards needs no reinforcement to be structurally sound, while an end-grain butt joint almost always does.