Stats

HP
HP (Colloquially believed to be Hit Points, Health Points, or Hit Poodles, as the Dog Salad states) is your character's vitality and ability to sustain attacks. This starts at 20, and only increases if you are on a Genocide or Neutral route when you kill monsters and level up. On a Pacifist route however, it can temporarily rise above 20 when sleeping at an Inn.

When your Soul is hit by a monster's attack, your HP is reduced based on the monster's Attack stat. When your HP reaches 0, the player's soul shatters, and the player is sent to the GAME OVER screen.

HP can be restored through various consumable items or through interacting with a save point, and can be risen past its maximum by sleeping at the inn.

Attack
Attack (or AT) is a stat that measures the rate of which you do damage. This is measured for both you and monsters. AT can be increased by using different weapons, but also by using certain ACT options and eating certain items. AT can be accounted for with a large amount of Defense.

Defense
DF is a stat used in Undertale that affects both you and enemies. It stands for Defense. The greater the number of DF a character has, the less damage you take. You can change your defense by either gaining LOVE or by wearing Armor. Certain ACT commands and Consumables will also affect your DF, as well as maybe affecting the enemies DF.

EXP
EXP, though often known to stand for "Experience Points," truthfully means "Execution Points," as explained by Sans in the Final Corridor. Getting enough EXP can increase your LV, or "Level of Violence." This makes it easier for the player to kill monsters and gives them more HP.

In the Pacifist Route the player does not gain any EXP because they do not kill any monsters. However, during a Genocide Run the player gathers a mass of EXP, resulting in a very high LV rank. This is not necessarily a good thing if the reactions of NPCs are anything to go by.

LOVE
LOVE (also known as LV and Level Of Violence) is the leveling system in Undertale.

As the player's LOVE increases, their HP, attack and defense stats increase.

The player starts the game at LV 1. Earning EXP from killing monsters increases LV. If the player intends not to kill anyone, their EXP will remain at 0 and their LV at 1.

In the demo, EXP can be gained from hurting monsters alone - killing them is not required. However, in the full game, EXP can only be gained through killing.

Main Story
Flowey, in the beginning, tells you that LV stands for LOVE. He tries to use this to trick the player into walking into his bullets which he tries to fool you with the name 'friendliness pellets'.

On Neutral and Pacifist runs, Sans reveals what LOVE and EXP stand for in the Last Corridor; he explains that the term LOVE is an acronym for Level Of Violence, and that EXP stands for Execution Points. As each of these increase, then so does the character's proficiency in murder as well as the ability to emotionally separate themself from the acts that they are committing.

On a Pacifist run, Sans states that the player never gained any LOVE, but has instead gained love.

INV
Short for "invulnerability". It's the number of invulnerability frames you are awarded after being hit by an attack.

The Cloudy Glasses,Torn Notebook and Temy Armor increase this amount, otherwise it doesn't change.

SPEED
This stat determines the speed at which your Soul moves around the Bullet Board.

Sea Tea increases this, but certain ACT options will increase/decrease this stat.