Beta Flash

In manga, betafura ベタフラ, or "beta flash," beta furasshu ベタフラッシュ, is a white flash of light in a black background.^(tonbi.jp)Traditionally, it's rendered by drawing focus lines and then filling the outer area with black ink.^{(dic.pixiv.net)} The term beta ベタ refers to the black areas in manga.

Manga: Assassination Classroom, Ansatsu Kyoushitsu 暗殺教室 (Chapter 1, 暗殺の時間)

Usage

The beta flash is used in a few different ways. Here are some of the common ones:

Speech Balloon

A beta flash can be used as speech balloon.

Typically, when the whole background of a panel is black, that means the time has frozen and the panel just shows the character's reaction, expression. With the beta flash balloon, the thoughts of the character in that instant can be conveyed too.

Manga: Komi-san wa, Comyushou desu. 古見さんは、コミュ症です。 (Chapter 2, 平穏です)

e!?
え！？
Eh!?

Generally the "inner thoughts" are going to be some internal comment remarking what absurd situation they have in front of themselves.

See also: tsukkomi ツッコミ.

Manga: Sakigake!! Cromartie Koukou 魁！！クロマティ高校 (Chapter 18, ブレード・ランナーズ・ハイ)

Context: Mechazawa Shin'ichi メカ沢新一, who is a robot, says he doesn't like machines and that humanity will end up being dominated by machines if we don't stop relying on them, prompting Kamiyama Takashi 神山高志 and Hayashida Shinjirou 林田慎二郎 to think:
sore wa hyottoshite gyagu de itteru no ka!?
それはひょっとしてギャグで言ってるのか！？
Could it be that [he] is saying that as a joke?

When black ink is used on the speech balloon, but not on the rest of the background, that's called an "uni flash," instead, and often contain the internal ~~screaming~~ thoughts of a character.

Manga: Gabriel DropOut, ガヴリールドロップアウト (Chapter 24)

Context: a character is walking around when all of sudden this girl shows up and...
chotto okane kashite kurenai?
ちょっとお金貸してくれない？
Won't [you] lend [me] a little money?
nandesuka kono hito!?
なんですかこの人！？
What's [up with] this [girl]!?
- She just asked money from a complete stranger!

Realization Mark

The beta flash is also used as a type of realization mark. When a character notices or deduces something, a flash is drawn coming out of their eye or cheek. This flash is drawn even in anime, as it works as a symbol.