Early voting can be either in-person at early voting stations, or absentee votes can be sent by post.
In-person early voting is common in Finland and Canada, while Switzerland, Germany and the UK accept postal votes.
For the United States election 2020, people are able to cast their ballots before November 3, either by post or in-person.
Early voting has steadily increased with each election year in the US. The number of people who voted early in the 2016 presidential election was more than five times the number who voted early in 1992.
And with the Covid-19 pandemic affecting everything this year, and some form of early voting under way in all 50 states, record numbers of people are seen casting their ballots ahead of time.
Donald Trump is among those who has voted early. He cast his vote in Florida on Saturday morning.
But the process hasn't been universally popular.
So who is voting early, and why? And what do some people have against it? The BBC explains it all.
How many people have voted early so far this year?
According to the US Elections Project, as of October 23, more than 53 million Americans had voted early - either by post or in person. This is already more than the total number of early votes cast in the 2016 election.
The site run by Michael McDonald, a professor at the University of Florida who specialises in early voting, counted the state with the highest number of early voters so far is Texas, where more than 6.3 million people have already voted - for context, that's almost two million more people than voted for Donald Trump in the state in the last presidential election, and just two million away from the total number of votes cast in Texas in 2016.
Texas is followed by California, where more than 5.8 million people have voted, and Florida, which has had more than 4.7 million early voters so far.
The number of people who've voted early so far is also already about 37% of the total number of people who voted in the election in 2016.
Who is voting early?
First of all, a lot of early voters this year are young.
According to the Tufts University Center for Information and Research on Civic Learning and Engagement (Circle), more than three million young people - classified as those aged between 18 and 29 - had cast early votes by October 21.
About two million of those young votes were cast in 14 battleground states, Circle adds. Polls have also shown that young voters could turn out in greater numbers this year than in the last election.
In 2016, turnout was just over 46% among 18 to 29-year-olds - while 71% of people older than 65 voted. This year, a Harvard Youth Poll suggests that as many as 63% of people the same age will vote.
More African Americans are voting early this year, too. According to analysis from data firm TargetSmart, more than six times as many black voters had cast early votes by October 18 this year, compared to the same point in 2016.
Why are so many people voting early?
One overarching reason is the coronavirus pandemic.
The traditional way of voting - thousands of people going to a single polling station on one particular day, queuing with other people for hours, and standing in polling booths that are in proximity to each other - is not particularly conducive to social distancing measures, particularly when not all states have mandated face coverings indoors.
So for this election, at least 30 states have made adjustments to make it easier and more accessible for people to cast absentee votes.
This includes, for example, removing strict requirements for people to have an excuse to have a postal vote, or allowing people to cite their concerns about Covid-19 as their excuse.
Some states have also implemented ballot drop boxes, so people don't have to go to a crowded polling station, or have offered to prepay postage on election ballots that are sent in by post.
Others, such as California and Colorado, have proactively sent all eligible voters mail-in ballots - a practice known as universal mail-in voting.
In North Carolina, the state's board of elections extended the deadline for postal ballots, saying that any ballots received up until November 12 would be counted - as long as they were postmarked on or before November 3.
Some states have also brought the start date for early in-person voting forward from last year - for example, Texas moved its start date forward by six days, from October 19 to 13.
Minnesota and South Dakota were the earliest states to begin early voting, with in-person voting starting 46 days before the election.
And while not all states have in-person early voting, those that don't are accepting early mail-in ballots.
Having said that, voting early doesn't necessarily mean voting easily.
In Georgia, for example, thousands of people waited for hours to vote at in-person polling stations.
And in Pennsylvania, the state supreme court ruled that ballots needed to be mailed in with a secrecy envelope hiding the identity of the voter. However, because this decision was announced so soon before voting opened, it caused concern that significant numbers of so-called "naked ballots" would be declared invalid.
There are also currently more than 300 lawsuits in 44 states concerning how absentee votes are counted, who is allowed to vote early, and how mail-in ballots are collected. Republican-run states say restrictions are necessary to prevent voter fraud, but Democrats say these are attempts at voter suppression.
How could this impact the election?
Perhaps the most obvious way an increase in early voting will affect the election is by driving up overall turnout.
Prof McDonald, who runs the US Elections Project, told Reuters news agency that he predicts that a record 150 million people could vote in this election - which would be about 65% of eligible voters in the country, and the highest turnout by percentage since 1908.
And according to Prof McDonald's data so far, the mail-in ballots are already leaning towards a Democrat majority.
However, this isn't a reliable indicator of who will win the election in the end.
President Trump launched a campaign against postal voting earlier this year, arguing that expanded mail-in voting would lead to the "most corrupt election" in US history and promoting claims that people would be able to vote more than once in the absentee ballot system. His dislike of postal voting may influence some of his supporters to wait until election day to cast their vote in person.
To clarify - there is no evidence of widespread fraud in the absentee voting system. In fact, the rate of voting fraud overall in the US is between 0.00004% and 0.0009%, according to a 2017 study by the Brennan Center for Justice.