A leaf is a leaf-node, one of the leaves in the BSP tree that is used to store your map. Each leaf-node is a small convex volume of space within the player-space inside the map's outer hull. Each leaf-node in the tree has a set of faces, which are flat (planar) surfaces which form portals where they meet the faces of other Leaf-Nodes in the map. This error is results when one of the leaf-nodes in your map meets too many other leaf-nodes, and therefore has too many portals. To fix it you can split the offending leaf-node up, or reduce the number of leaf-node it meets. Leaf-Nodes are created at the first q3map compile stage when the BSP Tree is generated. Put simply, the compiler arbitrarily splits the inside space of the map up into convex sections, using the faces of the brushes in the map to split along. To reduce the number of splits made you can simplify the brushes in the map. You can also make the compiler ignore certain brushes by tagging them as "detail" (Ctrl+M with selected brushes). To force a split in the map, you can use a Hint brush, which is an invisible nonsolid brush which only q3map can see and use (Hint brush = axial brush with all faces tools/hint).
Leaf has too many portalsThis error is results when one of the leaf-nodes in your map meets too many other leaf-nodes, and therefore has too many portals. To fix it you can split the offending leaf-node up, or reduce the number of leaf-node it meets. To reduce the number of splits made you can simplify the brushes in the map. You can also make the compiler ignore certain brushes by tagging them as "detail" (Ctrl+M with selected brushes). To force a split in the map, you can use a Hint brush, which is an invisible nonsolid brush which only q3map can see and use (Hint brush = axial brush with all faces common/hint). (Courtesy SmallPileofGibs) (QER)
Q. Leaf has too many portalsA. Generally means that you have a very complicated brush someplace probably with a lot of faces and is far too irregular and complex so the map wont compile so you need to make it less complex.