The HOI4 community uses a lot of acronyms and jargon. Here are the ones you'll run into most while reading these guides or talking with other players, explained quickly.

General

HOI4Hearts of Iron IV
The game's name, almost never written out in full.
Focus / focus tree
Each country's national decision tree: every activated focus unlocks events, bonuses or new options.
PPPolitical Power
The currency you spend to activate focuses, hire ministers, or make certain political decisions.
Ironman
A game mode with no manual saving — it autosaves and you can't reload a previous save. Required for achievements, but not recommended for your first game.
Historical / non-historical AI
An option when creating a game that decides whether the AI roughly follows the real World War II script or makes more unpredictable decisions.
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Economy and Research

Manpower
The pool of population available to recruit new divisions. It runs out if you recruit faster than it regenerates.
Production efficiency
A military factory's real output relative to its theoretical maximum. See the production guide.
XPExperience (army / navy / air force)
Points earned through research, training or combat, spent on creating equipment variants or upgrading templates.
MIOMilitary Industrial Organization
Manufacturer consortiums that improve with use, introduced in the Arms Against Tyranny DLC. See the DLC guide.

Combat

Combat width
The "space" a division takes up in a battle. See the dedicated guide.
OrganizationOrg
A division's resistance to staying in the fight. When it hits zero, the division retreats or collapses.
CASClose Air Support
Aircraft that bomb enemy divisions directly in combat. See the air warfare guide.
Command capacity
The number of divisions a general or field marshal can lead without penalty. See the military command guide.
Doctrine
Your nation's land combat philosophy (Mobile Warfare, Superior Firepower, etc.). See the doctrines guide.

Diplomacy and Occupation

World tension
A global gauge that determines how much room you have to take aggressive actions without penalty. See the diplomacy guide.
Faction
A group of countries that commit to defending each other in case of war.
Wargoal
The process — and waiting time — needed to declare war on another country "legitimately" within the game.
Compliance / Resistance
Occupation mechanics introduced in La Résistance: how much the population accepts your control, and how much it organizes to undermine it. See the DLC guide.