As part of the larger Fate/Stay Night universe, Fate/Grand Order throws a lot of random terminology that can be a bit overwhelming. This page gives the "plain speak" equivalents of all these jargon.

In Game

Master: You

Senpai: Used to refer to a senior member of an organization. In this context, it refers to you.

Servant: Units that you summon from battle, usually spirits based from myth and legends.

Noble Phantasm (NP): Special moves Servants can use when their NP gauge are fully charged.

Spirit Origin: Cards

Craft Essence (CE): Servant Equipment

Mystic Code: Master Equipment

Chaldea Gate: Mission Hub

Singularity: Story Chapter

Holy Grail: Story MacGuffin. Also a very limited resource for raising Servant level cap.

Spirit Origin: List of all seen and acquired Servants

Saint Quartz: Currency used for summoning 3* to 5* cards.

Friend Points: Currency used for summoning 1* to 3* cards.

Mana Prism: Shop Currency

Burning: Selling cards for mana prisms.

QP: Common currency used in leveling and upgrading Servants and Servant skills.

Ascension: Upgrading a Servant; increase level cap and unlocks new skills/portraits/dialogues.

Palingenesis: Upgrading a Servant with a Holy Grail; Also increases max level cap.

Ember/Fire/Blaze of Wisdom: Experience cards used to level up Servants.

Fandom

Waifu / Husbando: Favorite Servant (Female/Male)

Gems: Saint Quartz

Apologems: Free Quartz for when the servers are down.

Gacha: Summoning screen (Luck-based/ lottery system used for summoning)

Rolling: Summoning

10 Roll: A 30 Quartz roll that gives slightly better odds.

YOLO roll: a single Quartz or Ticket roll

Memeroll: a single Quartz or Ticket roll

Banner: Limited time special summoning opportunity with increased rate for featured Servants and CE.

Rate Up: Do not trust their lies.

Salt: Complaints

F2P: Free to play players

Whale: Players that pay for the server upkeep.

Reroll: Resetting the game by deleting game data (or reinstalling the game) for a chance at better Servants with the initial summons.

Leyline: Dragon Meridian CE

Nicknames

See this topic by /u/FileLoop for nickname origins.

Archer: EMIYA

Artemis Orion

Artoria: Altria Pendragon

Banana: Ibaraki-Douji

Cat: Tamamo Cat

Cockroach: Cu Chulainn

COOL: Gilles de Rais

Dog: Cu Chulainn

Eggplant: Mash Kyrielight

Gudako: Female Protagonist

Gudao: Male Protagonist

Herc: Heracles

Iri: Irisviel (Dress of Heaven)

Iskandar: Both Iskandar and Alexander

JAlter: Jeanne d'Arc (Alter)

Jannu: Jeanne D'Arc

Kiritsugu: EMIYA (Assassin)

KIRA KIRA: Marie Antoinette

Kiyokiyo: Kiyohime

Ko-Gil Kid Gilgamesh

Mama: Minamoto-no-Raikou or EMIYA

Moedred: Mordred

Nobu: Oda Nobunaga

REGEND: Sasaki Kojiro

ROMA: Romulus

Saber: Altria Pendragon

SAlter: Altria Pendragon (Alter)

Savior of France: Sasaki Kojiro

Shishou: Scathach

STELLA: Arash

Sumanai: Siegfried

Umu: Nero Claudius

Waver: Zhuge Liang

Acronyms

* Description
A Arts Card
AoE Area-of-effect Damage
AP Action Points; Stamina
B Buster Card
CE Craft Essence; Equipment
CAH Cursed Arm Hassan
CN Chinese/Taiwanese Server
DW Delightful Work; Game Developer
EX Extra Card
F2P Free to Play Players
FP Friends Points; For Summoning
JP Japanese Server
MLB Max Limit Break; Combining 5 CEs for upgrades to effect
MP Mana Prism; Shop Currency
MXH/MHX Mysterious Heroine X
NA North American Server
NP Noble Phantasm; Special Move
NP# Noble Phantasm levels by combining duplicate Servant cards
P2W Paid-to-Waifu players
Q Quick Card
QP Quantum Piece; general currency
SQ Saint Quartz; paid summoning currency.
ST Single Target Damage
TM Type Moon; Franchise Creator