A century is a period of 100 years.
The 21st (twenty-first) century is the current century of the Anno Domini era or Common Era, in accordance with the Gregorian calendar. It began on January 1, 2001, and will end on December 31, 2100. It is the first century of the 3rd millennium. It is distinct from the century known as the 2000s which began on January 1, 2000 and will end on December 31, 2099
The world population was about 6.1 billion at the start of the 21st century and reached 7.7 billion by January 2019. It is estimated to reach about 8.6 billion by the year 2030 , and 9.8 billion by the year 2050.
Economically and politically the United States and Western Europe were dominant at the beginning of the century; by the 2010s China became an emerging superpower and by some measures the world's largest economy. In terms of purchasing power parity India's economy became larger than that of Japan around the year 2011.
According to World Economic Forum, 65% of children entering primary school will end up in jobs or careers that currently do not yet exist. According to the United Nations World Urbanization prospects, 60% of the world's human population are projected to live in megacities and megalopolises by 2030, and 70% by 2050. By 2040, more than 5 times the current global GDP are expected to be invested in urban infrastructure and its use.
War and most kinds of crime and violence have declined considerably compared to the 20th century. Poverty is still widespread globally, but fewer people live in the most extreme forms of poverty. In 1990 nearly 36 percent of the world's population lived in extreme poverty compared to only 10% in 2015.
Life expectancy continues to increase and child mortality continues to decline. A baby born in 2016 can on average (globally) expect to live 72 years; 26 years longer than the global average of someone born in 1950. Ten million Britons (16% of the population of the United Kingdom) are expected to live to 100 or older.  However climate change remains an extremely serious concern; being described by UN Chief António Guterres as an "existential threat" to humanity. 
Advances in technology such as ultrasound, prenatal genetic testing and genetic engineering is changing the demographics and has the potential to change the genetic makeup of the human population. Because of sex selective abortion, fewer girls have been born in the 21st century (and since 1990) compared to in past centuries, mostly because of son preference in East and South Asia. In 2014 only 47 percent of Indian births were of girls.  This has led to an increase in bachelors in countries such as China and India. The first genetically modified children were born in November 2018 in China, beginning a new biological era for the human species and raising great controversy.
Anxiety  and depression  rates are rising in the United States and many other parts of the world. However, suicide rates have fallen in Europe and most of the rest of the world so far this century, declining 29% globally between 2000 and 2018, despite rising 18% in the United States in the same period. The decline in suicide has been most notable among Chinese and Indian women, the elderly and middle-aged Russian men.
Telecommunications in the early 21st century are much more advanced and universal than they were in the late 20th century. Only a few percent of the world's population were Internet users and cellular phone owners in the late 1990s; as of 2018 55% of the world's population is online and as of 2019 an estimated 67% own a cell phone.  In the 2010s artificial intelligence, mostly in the form of deep learning and machine learning became more prevalent, and is prominently used in Gmail and Google's search engine, as well as in banking, the military and other niches. In 2017 14% of the world's population still lacked access to electricity. 
In 2001 Dennis Tito became the first space tourist, beginning the era of commercial spaceflight. Entrepreneurs such as Elon Musk and Richard Branson are working towards commercial space exploration, colonization and tourism, and China and India have made substantial strides in their space program. On January 3, 2019 China landed a robotic spacecraft on the far side of the Moon, being the first to do so