beta-me ベタ目

In manga, beta-me ベタ目 are eyes drawn completely black, without highlight or distinct iris and pupil, or eyes draw in a single color other than black other than black, or eyes drawn without a highlight.

It's also spelled beta-me ベタ眼.

Character: Shinkouhyou 申公豹
Manga: Houshin Engi 封神演義 (Chapter 8, 序章の終わり)

Basically, in manga beta ベタ means an area filled with black ink, but eyes drawn pure black in manga are sometimes drawn with highlights in close-up panels, or in anime adaptations, so the term is kind of vague.

Examples

For reference, some examples of beta-me ベタ目.

An actual black one:

Character: Shinkouhyou 申公豹
Manga: Houshin Engi 封神演義 (Chapter 8, 序章の終わり)

In color:

Character: Shinkouhyou 申公豹
Anime: Hakyuu Houshin Engi 覇穹封神演義 (Episode 1)

Another example:

illumi Zoldyck イルミ゠ゾルディック, example of beta-me ベタ目.

Character: illumi Zoldyck イルミ゠ゾルディック
Manga: Hunter x Hunter (Chapter 26, 決戦前夜)

Character: illumi Zoldyck イルミ゠ゾルディック
Anime: Hunter x Hunter (2011) (Episode 15)

A white one:

tanshoku
単色
Single-color.

Kuroki Tomoko 黒木智子, example of beta-me ベタ目.

Character: Kuroki Tomoko 黒木智子
Manga: Watashi ga Motenai no wa Dou Kangaetemo Omaera ga Warui! 私がモテないのはどう考えてもお前らが悪い！ (Chapter 7, 何気ない日常生活を送る)

chira
ちら
*glimpse* (looks to the side)

Which in color becomes two different shades of green:

Kuroki Tomoko 黒木智子, example of character with dark circles under eyes.

Character: Kuroki Tomoko 黒木智子
Anime: Watashi ga Motenai no wa Dou Kangaetemo Omaera ga Warui! 私がモテないのはどう考えてもお前らが悪い！ (Episode 1)

And an example of an eye without highlight.

Morita Mayu 森田真由, example of betame ベタ目.

Character: Morita Mayu 森田真由
Manga: Morita-san wa Mukuchi 森田さんは無口 (Chapter 1)

In color:

Character: Morita Mayu 森田真由
Anime: Morita-san wa Mukuchi 森田さんは無口 (Episode 1)

Note that, typically, eyes are drawn without highlights in manga and anime to represent the mental state of a character, like when they're traumatized, and are known as "vacant eyes," utsuro-me 虚ろ目, and various other names.

The beta-me ベタ目, despite looking identical, doesn't carry any symbolism. The character isn't traumatized or anything, that's just how their eyes are by default, it's part of the design of the character.