Definition - What does Downvote mean?
A downvote is an action that a user can take on the Reddit website (and in some other user interfaces) that is used to signal disapproval or try to downgrade a post and its content. Some formal definitions of downvote refer to “voting against a cumulative tally of popular support.” Downvote is the opposite of upvote, which represents support for something. On Reddit, upvote and downvote user events are tabulated, and have an effect on how content is represented on the Reddit website.
Techopedia explains Downvote
As an early form of online bulletin board service, Reddit contains massive amounts of individual posts. Reddit users can go in and upvote or downvote, and change the prominence of a particular post. However, some confusion about the meaning of upvoting and downvoting has led to some specific instructions on some areas of Reddit. There has been a debate about these user controls in Reddit, and others, such as the "Like" button on Facebook. In some cases, there is some ambiguity about what these user events mean, and sometimes, people jump onto the system to comment and explain their actions. It is all part of creating transparency in the digital world where icons and indicators can seem unclear in certain situations.
In addition, both terms, upvote and downvote, have been associated with other Reddit slang such as upboat, upgoat, downboat, and downgoat. These slang terms originate from users making the mistake of typing the letters b or g on the keyboard instead of v when typing the terms upvote and downvote.