Skill

Skills are what magical, physical and other abilities are called in this world. The effect of these skills are wide-ranging. They can be practical, such as cleaning, constant use, such as improving perception, combat and other various things. On a technical and fundamental level, however, it not known what exactly the nature of skills are. Rimuru himself had asked this question once. Even so, it can deduced that skills are simply a mechanism that grant special characteristics to an individual - physical or magical.

Some skills can be learnt through extensive work in that field, some are innate to specific species, while others are specific to an individual.

Combining and Evolving Skills
Skills will also evolve with the level of the individual. As an individual levels up and evolves so will their associated skills. This is the most basic way of improving one's skills.

Naturally, skills can also evolve with practice or proficiency of the specific skill. They can be also be combined to form new and better skills with sufficient and deep knowledge regarding said skill. A magnified effect of this is seen with Rimuru's Great Sage (and its evolved forms) where one of its effects is to gain deep knowledge about skills more quickly and efficiently.

Skill Components
All skills from Extra Skills and above, have three components: abilities, effects and skills. Abilities define the general formula for a skill, for example, whether a skill embodies the concept of wrath or fire etc. A simple generalization. Diving further in, we get effects, which are passive abilities, and skills, which are active abilities.

It is safe to assume that all skills from and above Extra Skills act as 'packages' and 'strengtheners' for abilities inferior to but make up themselves.

Daily Skills
These are skills that can be continuously kept on with little to no use of MP, such as perception and enhancement skills that are ingrained into the body. For example:
 * Magic Perception
 * Heat Detection
 * Auditory Perception
 * Herculean Strength

Extra Skills
These are skills that are special, in terms of use and versatility. Extra Skills can evolve into Unique Skills. Some examples are:
 * Black Flame
 * Black Lightning
 * Barrier

Battle Skills
Skills that are meant to be used in fights. Although this is what the name implies, some, if not many, of the mentioned skills in the novel seem to still have use outside of battle. For example:
 * Sticky Thread
 * Telepathic Communication
 * Cloning

Unique Skills
Special skill that come with a variety of subsequent abilities. They're like a package several abilities enclosed as one skill, unique to an individual. Examples include:
 * Great Sage
 * Gluttony

Ultimate Skills
The pinnacle stage of all skills, hence 'ultimate'. They can arise from Unique Skills evolving or combining appropriate skills together. Examples include: Similar to Unique Skills, Ultimate Skills also come with several abilities packed into one. Among the Ultimate Skills one can achieve, the strongest among those are the Sin Series and Virtue Series Ultimate Skills that are capable of removing the racial limitations of Demons and Angels respectively, if one gather all seven of them each.
 * Gluttonous King Beelzebuth
 * Wisdom King Raphael
 * Investigation King Faust

In a much rarer case, Ultimate Skills can reach a further progression by evolving into a God Series Ultimate Skill, usually through training the already existing Ultimate Skill to its limits, or combining Sin and Virtue Series skills.

Ultimate Skills also have the ability to develop an ego, although it is not precisely known how. It is somewhat similar to having a computer granting you certain abilities, while at the same time, it learning about feelings and ego from its user. If the ego of an Ultimate Skill fully ripens, it can evolve into a Manas.

Ultimate Gifts
A degraded form of Ultimate Skills. Examples:
 * Gourmet King Beelzebub
 * Book of Magic Necronomicon
 * Pierrot Star

Manas
When a skill is no longer purely a skill, but an entirely different existence. A Manas is created when an Ultimate Skill (most probably a Sin or Virtue Series) gets an ego and a name, which solidifies its individuality.

Resistances
Although technically not 'skills', these abilities that provide mitigation against physical, mental and magical effects, such as:
 * Thermal Fluctuation Resistance
 * Heat Attack Resistance
 * Paralysis Resistance
 * Pain Resistance
 * Poison Resistance