Game mechanics

Player mechanics

 * Aggro - Aggro determines the distance at which enemies will begin to attack the player.
 * Attack speed - Attack speed is handled similar to movement speed: additive bonuses except the cap is at 4 times original speed.
 * Breath meter - When a player's head is under water or honey, a Breath Meter appears directly above the player, representing the time the player has until they begin drowning.
 * Buffs - Buffs are special stats or visual bonuses which are granted to the player after consuming certain items. They last anywhere from 30 seconds to 45 minutes.
 * Critical hit - A critical hit has a small chance of happening every time the player hits an enemy. It deals double the damage of a normal hit.
 * Damage types - Damage types are categorized into melee, ranged, magic weapons, summoning weapons since 1.2 and, since 1.3.0.1, thrown weapons. Different armor types and consumables provide bonuses for different types of weapons, summarized below.
 * Death - Death happens when a character's health reaches zero. Items will be dropped depending on character difficulty and server settings if multiplayer. A status message will appear dependent on damage type.
 * Debuffs - Debuffs are temporary, negative status effects. Unlike Buffs, they cannot be canceled by right-clicking the icon. They can be canceled by using the Nurse NPC for coins.
 * Defense - Defense is the number that determines how much damage can be absorbed per hit. Damage the player takes is reduced by half their defense value, rounded up.
 * Difficulty - A character's Difficulty mode is selected during character creation, and only applies to the character being created. The Difficulty mode of a character determines what happens upon their Death.
 * Fall damage - Fall damage refers to the damage a player character sustains upon falling a large distance. It is one of the basic game mechanics.
 * Health - Health is the character's life count, represented by a meter of heart icons at the top-right of the screen. Each red heart shown represents 20 health. Each golden heart is 25 health.
 * Inventory - An Inventory is a set of slots where items appear, or can be placed for storage.
 * Knockback - Knockback is a basic mechanic whereby players and enemies are repelled in the opposite direction of an attack.
 * Mana - Mana is a resource used by the player when using Magic Weapons and Tools. The player's current and maximum Mana is shown as a vertical meter of blue stars on the right side of the screen.
 * Luck - Luck is a mechanic that influences the randomness of various activities.

Items

 * Autoswing - Autoswing is an item property in which a weapon is repeatedly used or activated when the it is held down. Nearly all weapons in  and  have autoswing.
 * Modifier - A Modifier applies changes to an item's statistics. All Weapons and Accessories have Modifiers applied at random when they are crafted, bought from an NPC, collected from Chests, or are picked up from drops; though one possible Modifier is no Modifier at all, which has a 25% chance of being applied.
 * Rarity - Rarity is usually considered as a measure of how difficult to obtain or how valuable an item is.
 * Use time - Use Time is a statistic that applies to Weapons, Tools and other useable Items, which determines the time that must pass, after use, before the same item or another item can be used again.
 * Value - Value is what is described as the worth in coins of an item. If sold by an NPC, it will also be the price the NPC sells that item for.
 * Velocity - Velocity is the speed of a projectile fired from a weapon. It is not to be confused with use time, which is how fast an item can be used (i.e. fast, average, slow).

NPC mechanics

 * AI - The AI (or artificial intelligence) is the behavioral pattern exhibited by an NPC, Enemy, or Critter. For instance, the Caster AI will always warp, shoot three times, pause for three seconds, and then warp again.
 * NPC spawning - NPCs, Enemies, and Critters will spawn off camera and behave according to their AI. The rate at which they are spawned, the maximum number, and the type spawned are all dependent on the time of day, biome, the tile type of the ground, and other factors.
 * NPC drops - Most enemies will drop coins when they are killed, and many also have a chance of dropping items as well. Several town NPCs also have drops.

World

 * Day and night cycle - The day and night cycle refers to the rising and setting of the Sun and Moon, and the ways this can affect the world.
 * Expert Mode- Expert is a world difficulty setting chosen during world creation. Once the world has been generated, this setting cannot be changed for that world.
 * Master Mode - Master is another world difficulty setting with the same rules as expert, but generally harder enemy and boss AI and stats.
 * Journey Mode - Journey is another world difficulty setting, where the player has more control over the world (e.g. time, weather etc)
 * Lighting mode - The lighting mode determines the way the light from all sources renders in the World. It is set from Terraria's settings menu, in the "video" sub-menu.
 * Map size - Map size refers to the size of a Terraria world. The sizes available depend on the platform used.
 * Moon phase - The Moon moves through a cycle of eight different phases, with the next consecutive phase appearing each night. The current moon phase is indicated by the amount of it that is visible each night.
 * Music - Terraria has exactly 88 tracks as of 1.4.0.5. Each one will play and loop endlessly while the player is in a specific biome or while certain events are taking place in that precise moment.
 * Spawn - The world spawn point is where players appear in a newly created world or when joining a multiplayer server. The world's spawn point will be on a surface within five blocks of the horizontal middle of the world.
 * Evil biome - Each world is generated with either The Crimson or The Corruption. On, the player can choose the biome during world creation.

Trivia
Some game mechanics affect the Razer Keyboard. Drowning will cause the keyboard to turn blue slowly, rising to the top. Going to a different biome will change the keyboard color. Low health will cause the "H" key to begin blinking, unless the player has no healing potions or the player is suffering from the Potion Sickness debuff.