Chichi Bukuro is an emerging genre of expressing body line especially breasts while the character is wearing cloth.
If you imagine the cloth being wet and sticking to the breast. That is the form of Chichi Bukuro.
For example, if a woman having big breast wearing one-piece dress in reality, the textile will simply fall straight down from the edge of breast and will look fat. However, Chichi Bukuro allows the cloth to stick to the body line just like Fujiko Mine's body suit.
It is possible in 2D, but to replicate Chichi Bukuro in reality, the cloth has to be made like a bag to fit to the breast. Therefore, such expression is called Chichi Bukuro (Breast Bag).

It was a word that naturally appeared in H-game industry. One of the most representative game that used Chichi Bukuro expression earlier was Kamikaze Explorer /g/370943/77deb467cc/
As well as the characters' cute looking, Kamikaze Explorer has great erotic expression. It ended up a massive sale of 40,000. The expression of characters' breasts being obvious even though the characters are wearing school uniform, made a sensation of a word Chichi Bukuro between users and that was the beginning of the trend.
Since the big hit of Kamikaze Explorer, Chichi Bukuro became a popular expression in 2D breast.

On the other hand, there are users who're negative to Chichi Bukuro expression because of its lack of reality. But nowadays, Chichi Bukuro expression is quite a normal. Chichi Bukuro made it possible to express the breast line with any type of cloth that used to be expressed only with school swim suit or tight body suit.

I collected some images of breasts that will most likely be fit into the definition of Chichi Bukuro.
Here's some tips for judging Chichi Bukuro or not(it is my personal view, not applying to everyone).

[NOT Chichi Bukuro]
1, Having some sort of support on under bust to lift up(corset, obi belt, belt, draping,arms, lying etc)
2, Being wet.

[NOT QUITE Chichi Bukuro]
1, Although breast line including under bust is fitting, there's an expression of tightness or wrinkles on cleavage(cleavage is not fitting to breast).

[YES OR NO]
1, Body suits. It is impossible for any body suit material to fit to cleavage to separate the each breast(the material has to be very very soft), but this kind of body suit has become an established genre of clothing. So it can not be clearly judged.

[DEFO Chichi Bukuro]
1, Although wearing cloth, breast look like body painting(excluding body suit).
2, Breast is lifted up unnaturally without any support.
3, Cleavage is separated without any support.
4, There's no or very little expression of tightness or wrinkles.

Enjoy!

I'll take your word for it, and thank you for the lesson.

Base +25, mojomunkeez +16, ziom236 +7, The Really Cool Guy +9, Gitami +14, tzur +10, Elvang +6

I'm REALLY likin'it. I gueess I'll have to buy some "breast-bag clothes" to my girlfriend one of these days...

Base +8

Did I just learn something on a H-Site?

Now, to find a way to subtlely brag about it to my friends & familly.

Base +10, mojomunkeez +16, hZi +6, udizzle +3, The Really Cool Guy +9, Guggy +8, Elvang +6

Finally, empirical evidence! Time to go get some Under Armour shirts from Big 5 and do some tests.

Base +8

The More You Know.

Base +9, Laharl_EAC +9

Down-vote the non-H tag please. 26 itself disqualifies it :P

Base +15

Very good gallery!

Base +5

artist 30 and 59 pic?

Base +6