What is Unique visitors?

In digital marketing, the notion of unique visitors is one of the most important, however different authoritative services define it in their own ways, which may be misleading.
What causes even more confusion, they count unique visitors in their own ways.
Let us figure out the real meaning of the term, the way it is defined by various platforms, and its practical use.

Definitions

Unique visitors is an estimate of distinct users your website (or its specific page) has had within a given period.
It is opposed to the “number of visits” and the “number of pageviews” that count each URL request.
The problem is, at times is it a bit harder to count users who have hit your website than people who have showed up at your party. Because physical bodies present at the party are easy to count just looking at them, while website users are represented by requests from their browsers to your server.
Users may leave your website and then return, switch networks, devices, browsers, but what you need to know is the actual person’s journey through your website, because at the end of the day it's a human being who takes decisions and pays for your products. If it is one person you want to track him or her as one, no matter how many devices and browsers he or she uses to confuse you;)
The web analytics algorithms are built and constantly improved to recognize each individual user by browser, by location, by behavior, by device, with cookies, without cookies, etc. All in all they manage to distinguish 99% of users and count each of them only once, thus ensuring considerably reliable estimates.

According to Google, unique visitors are simply called “users”. Their uniqueness is determined by the user identificator that each user is assigned with and tracked across all devices, browsers, sessions, heaven and earth. The timeframe is flexible.
According to Alexa, uniqueness is determined by the user’s Alexa account, and the timeframe is one day.
According to Adobe Analytics, uniqueness is determined by cookies, and the timeframe is flexible.

The practical use

Perhaps you have already reflected on the connection between the “visits” and the “unique visitors”. Dividing the former by the latter you can learn the average visiting frequency per user. Or, in case we’re speaking in Alexa terms, the average number of pages a user views within a day.
Knowing these metrics and their dynamics, you can get a better understanding of how your visitors engage with your website, how often they return, and how many visits it takes a user to either convert or change their mind.
As an example of practical use, you may notice a segment of users who enter your website from time to time, open a certain product page, but leave without purchasing it. Such behavior reveals their interest for your product, but something is missing. Either they need more time (and more attention on your behalf) to convert, or they face some trouble (a typical case is a feedback form that is too complicated or irrelevant, another is some functionality that simply doesn’t work as it is supposed to).

Besides your own analytics the number of unique visitors helps ranking platforms, primarily Alexa, to evaluate your website. Alexa rank considers daily unique visitors and pageviews data. Google also considers your unique visitors when ranking your website against its competitors by the search query.
Big unique visitor traffic helps you to climb up the search result pages and get even more unique visitors.

To sum it up, unique visitors is the number of your website’s individual users. Depending on the results you need, you can figure out what rank you want to be high on (e.g. on the Google search results page to a specific query) and take actions discussed in this article to improve your position on that rank.