Game mechanics

Player mechanics

 * Attack Speed - Attack speed in Terraria is handled like movement speed: additive bonuses except the cap is at 4 times original speed.
 * Breath meter - When a player's head is under Water, 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 2 minutes to 15 minutes.
 * Critical Hit - A critical hit has a small chance of happening every time you hurt an enemy. It deals double the damage of a normal hit.
 * Damage types - Damage types are categorized into melee, ranged, magic weapons and since 1.2, summoning weapons. Different armor types and consumables provide bonuses for different types of weapons, summarized below.
 * Death - Death is what happens when your character's health reaches zero. A message will be sent dependent on how you die.
 * Debuffs - Debuffs are temporary, negative status effects. Unlike Buffs, they cannot be cancelled by right-clicking the icon.
 * 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.
 * Inventory - An Inventory is a set of slots where items appear, or can be placed for storage.
 * Knockback - Knockback is a basic mechanic in Terraria, whereby players and enemies are repelled in the opposite direction of an attack.
 * Mana - Mana is a resource used by the player when wielding Magic Weapons. The player's current and maximum Mana is shown as a vertical meter of blue stars on the right side of the screen.

Items

 * Autoswing - Autoswing is an item property in which a weapon is automatically swung when the key/button activating it is held down.
 * 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 monetary worth 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. For instance, the Caster AI will always warp, shoot three times, pause for three seconds, and then warp again.
 * NPC Spawning - In Terraria, monsters will spawn off camera and pursue the player based on their AI. The rate at which monsters are spawned, the maximum number of monsters, and the type of monster spawned are all dependent on the time of day, biome, the tile type of the ground, and other factors.
 * NPC drops - Most monsters will drop coins when they are killed, and many also have a chance of dropping items as well.

World

 * Day and Night Cycle - The day and night cycle of the Terraria world refers to the rising and setting of the Sun and Moon, and the ways this can affect the world.
 * Lighting mode - The lighting mode determines the way 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. Upon creation, you can select either "small", "medium", or "large" for the size of the map you generate.
 * Moon phase - The Moon in Terraria 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 38 tracks as of 1.3. Each one will play and loop endlessly while the player is in a specific biome or while certain events are taking place.
 * 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 5 blocks of the horizontal middle of world.