One of the few Russian words that every American knows is космонавт, which of course is spelled ‘cosmonaut’ in English.¹ It is a perfectly regular noun that declines like this:

SgPl
Nomкосмонавткосмонавты
Accкосмонавтакосмонавтов
Gen
Preкосмонавтекосмонавтах
Datкосмонавтукосмонавтам
Insкосмонавтомкосмонавтами

The космо- part is from Greek κόσμος, which meant universe (and nowadays ‘space’), and νάυτης sailor, thus a космонавт is a universe sailor, i.e., a space traveller. In English cosmonaut and astronaut are diffferent only in that the word cosmonaut applies to people of the former Soviet Union or of the countries that succeeded it. Astronaut is more general. It sounds perfectly normal to say “Yuri Gagarin was a Russian astronaut,” but to say “Alan Shephard was an American cosmonaut” sounds ironic.

Первым космонавтом был Юрий Гагарин.
Yuri Gagarin was the first cosmonaut.

Валентина Владимировна Терешкова — первая в мире женщина-космонавт. (source)
Valentina Vladimirovna Tereshkova was the first woman astronaut in the world.

— Я слышал, что ты хочешь быть космонавтом.
— Ты не расслышал. Я хочу быть трактористом.
“I heard that you wanted to be a cosmonaut.”
“You misheard. I want to be a tractor driver.”

Полёты в космос — опасная и сложная профессия. С начала эры космических полётов в космосе и при подготовке к космическим полётам на Земле погибли двадцать два космонавта. (source)
Space flight is a dangerous and complex profession. Since the beginning of the era of space flight twenty-two astronauts have died [during flights] or during flight preparation on Earth.

In 1995 the French decided that astronaut was much too American a word to describe their own astronauts, so they coined the word spationaut. I'm rather amused at the attitude, particularly since the word astronautique was first used in French back in the 1920s, long before Americans decided on the word astronaut in the late 1959.² And then the Chinese, not to be outdone by the Americans, the Russians and the French, decided they needed their own word as well, so in 1998 the word taikonaut was coined from the Cantonese word taikong ‘space’ to refer to Chinese astronauts.³

Whew, can political correctness get much correcter? Now I have to distinguish cosmonauts from taikonauts from astronauts. And with great glee I can now call French astronauts “spacey-o-nauts.”⁴

¹ The other two Russian words every American knows are vodka and czar.

² See an interesting discussion of the orgin of the word on http://www.collectspace.com/.

³ For commentary on the usage of the word taikonaut see this Wikipedia article.

⁴ Note to Russian speakers: in colloquial American English ‘spacey’ means ‘inattentive’ or ‘irresponsible,’ so spationaut when deliberately mispronounced as spacey-o-naut sounds mildly derisive and may evoke a chuckle from American listeners.Original post blogged on b2evolution.