Adaptive force is a stat that grants champions either 
attack damage or 
ability power, depending on the current amount of bonus attack damage and ability power of the champion, as follows:
Higher Bonus AD → Attack damage- Higher AP → Ability power
1 point of Adaptive Force provides 0.6 bonus AD or 1 AP.
If the attack damage and the ability power of the unit are equal, the stat granted depends on the champion.
In most cases, the League of Legends Wiki omits adaptive force in favor of displaying the amount of AD or AP granted.
Attack damage and ability power granted by passive effects do not contribute towards determining whether attack damage or ability power is gained by adaptive force. For instance, if 
Veigar has 100 stacks of 
Phenomenal Evil Power and 10 stacks of 
Dark Seal, his adaptive force will be gained as ability power. However, if he purchases an item that grants attack damage as a stat, such as 
Pickaxe, then his adaptive force will be gained as attack damage despite having a higher value of ability power.
It is important to note that the stats granted by mythic item passives, 
Crown of the Shattered Queen's Safeguard, 
Rabadon's Deathcap's Magical Opus, and 
Sterak's Gage's The Claws That Catch are special cased to contribute towards the stat granted by adaptive force.
Sources of adaptive force bonuses
Champions abilities
Runes
Adaptive damage
A similar effect can be found on:
Adaptive Damage: Deals either physical or magic damage depending on your bonus statistics, defaulting based on the origin of the effect.
- Higher Bonus AD → Physical damage
- Higher AP → Magic damage
If the attack damage and the ability power of the unit are equal, the damage determined depends on the champion.
Sources of adaptive damage
Runes
Neutral Buffs
