"plainness" in the noun sense
1. plainness
the state of being unmixed with other material
"the plainness of vanilla ice cream"
2. perspicuity, perspicuousness, plainness
clarity as a consequence of being perspicuous
3. plainness
the appearance of being plain and unpretentious
4. homeliness, plainness
an appearance that is not attractive or beautiful
"fine clothes could not conceal the girl's homeliness"
Source: WordNet® (An amazing lexical database of English)
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