Ecchi (エッチ etchi?, pronounced [et.tɕi]) is an often used slang term in the Japanese language for erotic fantasy and sexual innuendoes. As an adjective it is used with the meaning of "dirty", "naughty", "frivolous"; as a verb (ecchi suru), with the meaning to do something dirty, naughty, frivolous or to sleep together; or as a noun, to describe someone that is seen as ecchi. It is a synonym for ero (from Eros) and does not have such a harsh meaning as hentai.

The word is not only common in Japan, it is also used worldwide inside the fandom of Japanese media to describe sexual themes or undertones. While the word ecchi could mean anything from mild to insulting in Japanese language, it is used in Western culture to divide between pornography (hentai) and playful usage of sexualized imagery (ecchi).1 Works considered as ecchi do not show any sexual intercourse or primary sexual characteristics. Instead, it is up to the imagination of the viewer. Inside such media, it often goes along with fan service in a humorous way. These kinds of sexual themes or undertones can usually be found in comedic Shōnen/Seinen manga and harem anime.23

Fanservice is found in many anime, in many forms. When it appears to be a major focus of the show, then it's safe to assume you're watching Ecchi.

"Ecchi" (pronounced "EH-chee) is used by English-speaking anime fans to refer to soft-core pornographic or sexually suggestive anime and manga. This is entirely a western invention, as in Japanese, it is a catch-all term for all things sexual, not limited to porn and not really considered a "milder" term. The term "ecchi" in Japanese comes from the pronounciation of the letter "H" in English - "aitch" is rendered into Japanese syllabary as エッチ, which in turn is transliterated into the Roman alphabet as "ecchi". The letter "H" comes from the word "Hentai", but the term as currently used is not synonymous with Hentai.

Examples

12 Beast from the same author as Daily Life With Monster Girl and Deadline Summonner has this in spades, with the twist that they are (mostly) cute monster girls
Amaenaide yo!!
Asobi ni Iku yo!
B Gata H Kei
Battle Programmer Shirase
Baka To Test To Shoukanjuu
Boku Wa Tomodachi Ga Sukunai
Buttobi CPU could be considered a borderline case for either Ecchi or Hentai, though it is marketed primarily as the latter. It stands more or less in the same position as Ebichu — there's a lot of risque material, but it's too funny to be a straightforward Hentai.
Change123
Chobits
Chrono Crusade,
Colorful
Cube X Cursed X Curious
Daily Life With Monster Girl (AKA Monster Musume) has this in spades with cute monster girls.
Deadline Summonner from the same other as the above continues the trend.
DearS (Magical Girlfriend? Not necessarily ecchi. Magical sex slave? Ecchi.)
Elfen Lied — A subversion, the story as a whole is a drama/horror. Much of nudity is Fan Disservice and misleads the audience quite a lot.
Futari Ecchi — Although it does straddle the line between this and Hentai.
Gacha Gacha features plenty of boobs, panties and other lighthearted fetish material, there's even some nudity, although only limited to nipples.
Ghost in the Shell — Man-Machine Interface was pretty well fueled by this. The plot was not nearly as strong as the original manga and there seemed to be more panels featuring closeups of the Major's ass than anything else.
Go Dannar
Golden Boy
Gravion
Grenadier
Hagure Yuusha no Kichiku Bigaku
Hanaukyō Maid Tai
Hand Maid May
He Is My Master
Highschool of the Dead features Gainaxing on girls being torn apart by zombies, a girl cracking open human skulls with a wooden sword while wearing nothing but an apron and a thong, and one particular gratuitous sequence of bouncing in Bullet Time.
High School D×D
Higurashi No Naku Koro Ki kira - these OVAs much more fanservice centered than the original series.
History's Strongest Disciple Kenichi
Hyakuen
Idol Pretender
Ikki Tousen
Inumimi
I"s - manga about a guy who would really like to date a certain girl.
Jungle De Ikou
Kangoku Gakuen
Kanokon. So much hypersexuality in one half hour.
Koihime†Musou jumps between shameless pantyshotting and contriving to get the girls into as many compromising positions as possible, often lampshading it and complementing it with surprisingly classy dialogue and references.
Kyo Kara Maoh
Kyoukai Senjou No Horizon Its use of the World of Buxom trope is enough to land it in this genre. Even without having the biggest breasts many of the woman are still well endowed.
Ladies Versus Butlers!
Love Hina
Mahoromatic
Mahou Sensei Negima!, by the same author as Love Hina.
Mai Otome Zwei. The rest of the Mai Hi ME franchise is fairly heavy on the Fanservice but doesn't focus nearly as much on it.
Maria†Holic
Mashiro-iroSymphony
MM
My Lovely Ghost Kana
Nagasarete Airantou
Nana To Kaoru
Nanatsu No Taizai
Need a Girl!
Ninin Ga Shinobuden (Simultaneously parodies this and plays it straight.)
Omamori Himari.
Please Teacher!, Onegai Twins, although these shows have a bit more serious content than most, especially the latter.
Ore No Imouto Ga Konnani Kawaii Wake Ga Nai
Oruchuban Ebichu (While the show featured material that could be considered explicit, the simplistic art style and goofy premise kept it from being taken seriously as Hentai.)
Panty & Stocking with Garterbelt
Princess Lover
Queen's Blade has no objection to showing nipples, and the proportions of the female characters cover every trope applicable to breasts.
Ranma ½
Rosario + Vampire (the anime; not so much in the manga)
Shinkon Gattai Godannar!!
SHUFFLE!, one of the best examples of Cerebus Syndrome in the genre (harem comedy turns dramatic halfway into the series. Let's not even get started on Kaede Fuyou snapping on Asa Shigure.
Heaven's Lost Property
Steel Angel Kurumi
Strike Witches
Tenchi Muyo!
Tengen Toppa Gurren Lagann - Guren Gakuenhen (The high school AU manga).
A case could certainly be made for the anime itself falling under this, especially Lagann-hen.
Tenjho Tenge
To Aru Majutsu no Index
They Are My Noble Masters.
To Love-Ru (Try looking for even one manga chapter without gratuitous nudity and/or underwear shots. The anime adaptation even has nudity and fanservice in the OP, ED, and eye catches!)
Chapter 48 is an exception, believe it or not. It's probably the only one, too.
Darkness manages to be more Hotter and Sexier than the original.
Ultimate Girls
Zero No Tsukaima
Underdog
Wagaya no Oinari-sama
The Witchblade anime surely delights in showing Masane's... ample assets with VERY LITTLE covering them. However, it's in many ways a subversion - as the actual appearance is quite sexy, what's she doing is absolutely not.
The Animesque comic Empowered. Sex scenes galore, but Godiva Hair and Scenery Censor are flat-out abused.
Sekirei is a pretty ecchi manga and anime series... including giant breasts, nearly naked scenes and lots of nosebleeds from the main male character.
Maicchingu Machiko Sensei was one of the Ur Examples — being a kids show that revolves around new and creative ways for the titular Machiko to lose her clothing and/or be sexually molested by her elementary school students.
Seikon No Qwaser takes this Up to Eleven, especially in the anime. This story crosses the line so many times you'd loose count in its depictions of large breasts, BDSM and milking, among other things. Yet it all actually aids the compelling plot and characters.