What is the meaning of Plain?

Flat, level.

Simple, unaltered.

Ordinary; lacking adornment or ornamentation; unembellished.

Of just one colour; lacking a pattern.

Simple in habits or qualities; unsophisticated, not exceptional, ordinary.

Having only few ingredients, or no additional ingredients or seasonings; not elaborate, without toppings or extras.

Containing no extended or nonprinting characters (especially in plain text).

Obvious.

Evident to one's senses or reason; manifest, clear, unmistakable.

Downright; total, unmistakable (as intensifier).

Open.

1. Honest and without deception; candid, open; blunt.

   Honest and without deception; candid, open; blunt.

   Clear; unencumbered; equal; fair.

   Not unusually beautiful; unattractive.

   Not a trump.

   Simply.

   Plainly; distinctly.

   An expanse of land with relatively low relief and few trees, especially a grassy expanse.

   A broad, flat expanse in general, as of water.

   Synonym of field in reference to a battlefield.

   Alternative spelling of plane: a flat geometric field.

   To level; to raze; to make plain or even on the surface.

   To make plain or manifest; to explain.

   A lamentation.

   To complain.

   To lament, bewail.

   Full, complete in number or extent.

   full

   full (not empty)

   full (not empty)

   plain (flat area)

   flat (not even or mountainous)

   plane

   full

   Source: wiktionary.org