Game mechanics

Player

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.
Death: Death happens when a character's health reaches zero. Items will be dropped depending on character difficulty and server settings in multiplayer. A status message will appear dependent on the cause of death.
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 is the damage a player character sustains upon landing on solid ground after falling a large distance.
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.
Luck: Luck is a mechanic that influences the randomness of various activities.
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.
Movement speed: Movement speed is one of the player's stats and affects how quickly they can travel around.

Attack speed: Attack speed is the rate at which a weapon can attack.
Buffs: Buffs are special stats or visual bonuses which are granted to the player after consuming certain items.
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: Damage is the number of health points subtracted from an entity's current health. Weapons are categorized into different damage types, these are melee, ranged, magic, and summon.
Debuffs: Debuffs are temporary, negative status effects. Unlike buffs, they cannot be cancelled for free at any time, and instead require paying the Nurse NPC for cancellation.
Defense: Defense is the number that determines how much damage can be absorbed per hit. It is more effective in Expert and Master Mode.
Knockback: Knockback is a basic mechanic whereby players and enemies are repelled in the opposite direction of an attack.
Armor Penetration: Armor Penetration is a hidden stat that allows the player to ignore a certain amount of enemy defense. Some weapons have a certain amount of it as an innate characteristic.

Autoswing: Autoswing is an item property in which a weapon is repeatedly used or activated when the ⚒ Use / Attack it is held down. Nearly all weapons in 3DS have autoswing.
Crafting: Crafting is the process of combining materials into new items.
Fishing: Fishing is an activity accomplished by using a fishing pole at a body of liquid while having bait in the player's inventory to get some game resources.
Modifier: A Modifier applies changes to an item's statistics. Weapons and accessories have a chance to have modifiers applied at random when they are crafted, appearing as loot in chests, or dropped by enemies.
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 usable items, which determines the time that must pass after use before the same item or another item can be used again.
Value: Value is the worth of an item in coins. 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.

Aggro: Aggro determines the distance at which enemies will begin to attack the player.
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 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.
NPC spawning: NPCs, enemies, and critters will spawn off-screen 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.

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.
Evil biomes: Each world is generated with either The Crimson or The Corruption. On Desktop, Console, and Mobile, the player can choose the biome during world creation.
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.
Spawn: The world spawn point is where players appear in a newly created world or when joining a multiplayer server. For a regular world, the world's spawn point will be on a surface within five tiles of the horizontal center of the world.
World difficulty: World difficulty is a world mode chosen during world creation and determines how challenging the gameplay is. There are four world modes: Journey Mode(Desktop, Console and Mobile versions), Classic Mode, Expert Mode(Desktop, Console and Mobile versions) and Master Mode(Desktop, Console and Mobile versions).
World size: World size refers to the size of a Terraria world. The sizes available depend on the platform used.

Golf: Golf is an in-game activity where players hit Golf Balls into Golf Cups using Golf Clubs.
Lighting mode: The lighting mode determines the way the light from all sources renders in the world. It is set from the settings menu, in the "video" sub-menu.
Music: Music will play and loop endlessly while the player is in a specific biome or while certain events are taking place.
RGB: RGB lighting effects are available for supported Razer, Logitech, Corsair and SteelSeries peripherals.
Status messages: Status messages display as chat lines in the lower-left of the game screen when certain conditions apply.