severity

1. The state or quality of being severe.
2. The act or an instance of severe behavior, especially punishment.

noun

1. The fact or condition of being rigorous and unsparing: austerity, hardness, harshness, rigidity, rigor, rigorousness, sternness, strictness, stringency, toughness. See easy/hard.
2. Exceptionally great concentration, power, or force, especially in activity: depth (often used in plural), ferociousness, ferocity, fierceness, fury, intensity, pitch, vehemence, vehemency, violence. See big/small/amount, strong/weak.

Definition: harshness
Antonyms: leniency

n

Definition: plainness
Antonyms: elaboration

n

Definition: sternness
Antonyms: geniality

n

Definition: strength, intensity
Antonyms: mildness

The noun has 3 meanings:

Meaning #1: used of the degree of something undesirable e.g. pain or weather
  Synonym: badness

Meaning #2: something hard to endure
  Synonyms: asperity, grimness, hardship, rigor, rigour, rigorousness

Meaning #3: excessive sternness
  Synonyms: harshness, rigor, rigour, inclemency, hardness, stiffness