Units may be divided into the major types of land, sea, air, and helicopter (helicopters are sufficiently different from air units to deserve special treatment). All units move from square to square on the map grid, in each step moving into one of the 8 squares around it, subject to many restrictions (exception: Airlift).
Land units move on land, and suffer the movement costs of terrain, but benefit from roads and railroads. Units are created with a specific number of movement points; a Diplomat, for instance, has 2 movement points. In each turn every unit gets to use all of its allotted movement points, although units that have received damage (see Combat) have a reduced allotment. A unit may not move into a square unless it has at least a fraction of a movement point remaining. For instance, in one turn a Diplomat may move from one Plains square to another, and to a third, but cannot then move to a fourth. Roads help by reducing movement costs to one-third of a movement point, regardless of the underlying terrain; on connected roads a Diplomat could then move six squares.
Land units cannot move on Ocean squares except when carried by sea units; they use land movement to move onto or off of their sea unit, but then should be told to Sentry until they are ready to disembark. While at sea they cannot attack or defend for themselves, but rely on the sea unit carrying them. If the sea unit is destroyed in battle, the passengers are destroyed as well. Land units leaving a sea unit may not attack in doing so; you may not directly assault an enemy city or unit from your ships (exception: Marines may attack directly from sea units).
On land, land units are subject to zones of control. The eight land squares around a land unit, city, or air or helicopter unit in a land square, are considered to be within the unit's zone of control. A land unit of one nation may not move directly from one enemy-controlled square to another, although there are three exceptions where zones of control do not apply:
Sea units move on Ocean squares, and into cities which are adjacent to Ocean squares. They may only be built in such coastal cities. Like land units, they have different movement point allocations based on their type; for instance, a Caravel has 3 movement points. Like land units, their movement is reduced when damaged by combat. Unlike land units, that basic movement ability may be modified globally by some Wonders of the World (Lighthouse, Magellan's Expedition), and by the knowledge of the advancement Nuclear Power. All movement of sea units, on all Ocean squares and into and out of all coastal cities, costs one movement point per square. They may not enter an enemy city, even if it is undefended, but they may attack enemy land units within cities. Some sea units may carry land units as passengers, up to a fixed limit.
Air units may move on any square, land or Ocean, that is not occupied by an enemy unit or city. They have movement points that are not reduced by damage, and use one point for every square. They have needs for fuel which can only be met by ending a turn in a friendly city or on a friendly Carrier (or a Submarine, for missile units). The number of turns on air unit can last without fuel varies by the unit type. Air units may not be used to explore Villages, nor may they capture an enemy city.
Helicopter units can move into any Square not occupied by an enemy unit, and their movement rate is not diminished by damage. They do not require fuel; however, they require maintenance. Every turn in which they do not end in a friendly city, they lose 2 hit points (see below on hit points) from wear and tear (unless you have the United Nations Wonder). They may enter and capture an undefended enemy city.
As noted above, many unit types have reduced movement when damaged. In addition to a movement point allocation, every unit type has a number of "hit points" indicating the unit's state of repair. In Combat units receive damage by losing hit points. Damaged units have their hit points restored gradually in every turn in which they do not move or attack. Typically this restoration is one hit point per turn; however a unit regains all of its hit points if: