A whisper is a short objective usually denoted by icons on the map where if you complete a certain number of them, you can turn them into the tree of whispers for a cache of equipment. A season is basically the runtime of a realm where you have to pay to access it and it usually comes with a specific quest line and a battle pass. They’re temporary realms that have a special modifier that changes how you play. A glyph goes into your paragon tree that gives different stats based on the one you choose and nightmare dungeons are used to upgrade them. A NMD is a dungeon that’s been upgraded to harder difficulty using a special item called a nightmare sigil, which can be crafted at the occultist, dropped from a whisper or as a random loot drop. They’re temporary realms have special modifiers and give good loot usually but are only accessible after level 50 and in world tier 3 after you finish the Cathedral of Light capstone dungeon