Note: This glossary is a work in progress. More information over at /r/badredman

Area Denial
Preventing your opponent from occupying a certain area. Usually done by filling space with a lingering AoE like Founding Rain of Stars or most Magma Sorceries. Area denial tools are generally also zoning tools. With proper positioning, you can restrict your opponent’s movement to ensure they stay where you want them to be. (Credit to RidiculousFalcon)
Bonfire Duelist
Name for a player that sits at a Grace Point with Taunters Tongue active in order to duel invaders, often with an unfair advantage (Rune Arc, Summoning Blues, etc.). Video Explanation
Blue
A player that gets summoned to fight off invaders when the Host of the game has the White Cipher Ring active. Also known as “Free Health Packs” in the invader community.
Cheese
Coming…
Encirclement
A situation where you are surrounded by multiple opponents. Always try to be aware of your opponents positions and keep them in your view in a single line.
Gank
A group of players that specifically get together with the sole intention on ganging up on invaders. Usually making sure that they have every single advantage to defeat the invader (killing all enemies in the area, summoning over leveled phantoms, etc.)
Git Gud
[Urban Dictionary] American English, bastardized way of saying “get good”. A very short, succinct way of saying: Quit playing the incompetent victim; practice makes perfect; You can do better, so do better.
(Hard-) Swapping
Opening your equipment menu mid-combat to switch your current equipped weapon/talismans/armor-piece with one in your inventory. Mid-combat Weapon-Hard-Swap example
(Soft-) Swapping
Pressing the left/right d-pad to switch between equipped weapons. Opposite of Hard-Swapping.
Hard-Counter
Switching / Hard-Swapping to a weapon that counters the currently equipped weapon of your opponent.
Host Sniping
The process of trying to kill the Host first while ignoring other phantoms (Not sure if this is a common term).
Matchup
Coming…
Meta
Meta is a setup or strategy that is accepted by a large portion of the community to be the most effective. This includes weapons, infusions, rune level, spells, armor, and any part of a build. (Credit to sam-austria-maxis)
Neutral
The state when you and your opponent are keeping distance between each other (you are not in attack range of one another).
Reset to Neutral
Disengaging the opponent and going back to keeping distance. Something beginners tend to do after a successful hit. Instead of resetting to neutral, you want to pressure the opponent instead.
Panic Roll
Spamming the roll button after getting hit, either once or multiple times. This makes it easy for the opponent to roll-catch you, since it will produce a steady rhythm of rolls that can easily be predicted. One should learn to reaction roll instead. Panic Roll example
Phantom Range
Every weapon has a unique range and hitbox (see here). The hitbox of some weapons is bigger / longer than the in-game model. Network latency can also produce a phantom range effect (It seems like you were out of distance from the weapon model but still get hit).
Playing Bunker (or Fotress) / Camping
A situation where the host and their phantoms settle down for an extended period of time in a safe, defensive location within the level, often a choke point or a high platform or hill. (Credit to TheGreatZephyrical)
Priority
Priority is who hits first in an exchange between two players. (Thanks to Acal0wastaken for the clarification)
Pressure
Coming…
Reading / Reads
The process of trying to read / predict the behavior of your opponent and take according counter actions. Reads can also be done about the environment (how many opponents are there, how are they behaving, where is the next group of mobs, etc.).
Reaction Roll
Deliberate and calculated rolls. The opposite of panic rolling.
Roll Catching
Timing of an attack right after the opponent’s roll animation ends. Fundamental skill to learn. The timing of the roll catch depends on the used weapon / spells and network latency. Continuous roll catching example
Reverse Backstep
Reverse Step (also known as Ravioli step) is a technique that allows you to use the forward momentum of a backstep at any time (example). Steelovsky has a great video about it.
Spacing
The practice of controlling the space between you and the opponent (standing just out of attack range of your opponent and knowing how far your opponent’s attack can reach). One of the most fundamental skills to master if you are trying to get better at the game.
Sunbro
Sunbro is a colloquialism referring to gold phantoms. The term originated from Dark Souls’ Warriors of Sunlight, a covenant focused on jolly cooperation. Unlike white co-op phantoms, Warriors of Sunlight glow an incandescent gold color. (Credit to Iced Estus)
Turn and Burn
While running away from a group or single enemy, baiting them into a surprise attack by turning around suddenly. Usually using sweeping or AoE (Area of Effect) attacks like Flaming Strike. This is a great turn and burn example by u/tragic-thing
Trade / Trading Attacks
Hitting the opponent at the same time he hits you. It can sometimes be done deliberately if you know that your attack will do more damage.
Whiff Punish
When your opponent misses his attack, and you punish him for it. Spacing related.
Zoning
Zoning is about controlling space. Proper positioning is an important aspect of effective zoning. To zone is to dictate where your opponent can and can’t go, lowering their options in the fight and giving yourself an advantage against them. (Credit to RidiculousFalcon)