Some people like to browse Pixiv for sexy art by Japanese artists. However, the overwhelming majority of content on Pixiv is described, and tagged with Japanese words. So searching for a tag in English will not yield many results.
So, here is a list of Japanese words you can use to search Pixiv for content you enjoy.
GENERAL:
へんたい - Hentai (This tag is not necessary for pictures tagged R-18 or R-18G)
無修正 - Uncensored (specifically refers to lack of mosaics or bars over genitals)
エッチ - Ecchi (Pervy, but not outright pornographic)
かわいい - Cute/Kawaii
落書き - “Rakugaki.” Scribble/sketch.
原創 - Original (Your own characters/setting, not from a franchise or anime)
アナログ - Analog (Specifically refers to piece not being digital)
裸 - Naked
ちび - Chibi
GENRES:
ファンタジー - Fantasy (Genre. Swords and Sorcery, etc)
BL - Boys’ Love
やおい - Yaoi (male/male sexual situations)
少年愛 - Shounen Ai (male/male, but more emotional than sexual)
薔薇 - Bara (Male/Male, but much more masculine. Generally involves burly, muscular characters)
ホモ - Homosexual (usually more masculine. Lit. “Homo”)
触手 - Tentacles
ケモノ - Kemono, Furry (lit. “Beast Person”)
ケモホモ - “Kemohomo,” gay furry
レズビアン - Lesbian
NOTE: I have noticed that there doesn’t seem to be any notable tags for hetero/straight-oriented art, as this is considered the “default” type of hentai.
FEMALE-SPECIFIC:
少女 - Girl
姫- Hime/Princess (Young lady of high birth, not necessarily royalty)
搾乳 - Milking/Lactation
母乳 - Breast Milk
巨乳 - Big breasts
おっぱい - Breasts
少女 - Girl
筋肉娘 - Muscle Girl (kinniku musume)
長身女性 - Tall girl (Chõshin josei)
MALE-SPECIFIC:
少年 - Boy (“Shonen.” Refers to young male characters, from childhood to age 18. Teenager.)
男の子- Boy (similar meaning to above)
青年- Young Man (“Seinen,” term for young adults)
美少年 - “Handsome youth.” Characters who are implied to be as such, in-universe.
巨根 - Big penis (“kyokon,” usually on explicitly masculine subjects. Bara, etc)
包茎 - Uncut penis (lit. “Phimosis”)
男の娘 - Femboy/Trap (Otokonoko, lit. “girl-boy”)
女装 - Crossdressing/Drag (specifically, males crossdressing as females)
女装少年 - Crossdressing boy
射精 - Ejaculation (“Shasei,” the act of shooting sperm)
ペニス - Penis (referring to the actual body part)
TRANS / FUTA-RELATED:
ふたなり - Futanari (Hermaphrodite, has both genitals)
ふた×男 - Futa on Male (literally “futa x male”)
ニューハーフ - Newhalf (Japan’s term for “Transexual” or “Shemale.” Breasts, dick, balls, but no vagina)
BDSM-RELATED:
緊縛 - Bondage
ボンデージ - Bondage (katakana)
SM - S&M
調教 - Torture (lit. “breaking”)
ラバー - Rubber/Latex
ラバースーツ - Rubber/Latex bodysuit
ラテックス - Latex (katakana)
ボールギャグ - Ball Gag
ギャグ - Gag
首輪 - Collar
目隠し - Blindfold
縛り - Shibari (Rope bondage)
緊縛 - Bondage (“kinbaku,” advanced rope bondage)
くすぐり - Tickling (lit. “Feathers”)
バイブ - Vibrator/Vibration
マゾ - Masochist
陵辱 - Rape/forceful sex
レイプ - Rape/forceful sex (katakana version)
身体に落書き - Body writing (Things written/drawn on skin)