I have a link to the American Atheists website that answers this question very well. Anyone who needs a definition this is a good write up on defining the term.

https://www.atheists.org/activism/resources/about-atheism/

"Atheism is one thing: A lack of belief in gods.
Atheism is not an affirmative belief that there is no god nor does it answer any other question about what a person believes. It is simply a rejection of the assertion that there are gods. Atheism is too often defined incorrectly as a belief system. To be clear: Atheism is not a disbelief in gods or a denial of gods; it is a lack of belief in gods.
Older dictionaries define atheism as “a belief that there is no God.” Clearly, theistic influence taints these definitions. The fact that dictionaries define Atheism as “there is no God” betrays the (mono)theistic influence. Without the (mono)theistic influence, the definition would at least read “there are no gods.”

Atheism is not a belief system nor is it a religion.

While there are some religions that are atheistic (certain sects of Buddhism, for example), that does not mean that atheism is a religion. To put it in a more humorous way: If atheism is a religion, then not collecting stamps is a hobby.
Despite the fact that atheism is not a religion, atheism is protected by many of the same Constitutional rights that protect religion. That, however, does not mean that atheism is itself a religion, only that our sincerely held (lack of) beliefs are protected in the same way as the religious beliefs of others. Similarly, many “interfaith” groups will include atheists. This, again, does not mean that atheism is a religious belief.

Some groups will use words like Agnostic, Humanist, Secular, Bright, Freethinker, or any number of other terms to self identify. Those words are perfectly fine as a self-identifier, but we strongly advocate using the word that people understand: Atheist. Don’t use those other terms to disguise your atheism or to shy away from a word that some think has a negative connotation. We should be using the terminology that is most accurate and that answers the question that is actually being asked. We should use the term that binds all of us together.
If you call yourself a humanist, a freethinker, a bright, or even a “cultural Catholic” and lack belief in a god, you are an atheist. Don’t shy away from the term. Embrace it. 

Agnostic isn’t just a “weaker” version of being an atheist. It answers a different question. Atheism is about what you believe. Agnosticism is about what you know.
Not all non-religious people are atheists, but…
In recent surveys, the Pew Research Center has grouped atheists, agnostics, and the “unaffiliated” into one category. The so-called “Nones” are the fastest growing “religious” demographic in the United States. Pew separates out atheists from agnostics and the non-religious, but that is primarily a function of self-identification. Only about 5% of people call themselves atheists, but if you ask about belief in gods, 11% say they do not believe in gods. Those people are atheists, whether they choose to use the word or not.

A recent survey from University of Kentucky psychologists Will Gervais and Maxine Najle found that as many as 26% of Americans may be atheists. This study was designed to overcome the stigma associated with atheism and the potential for closeted atheists to abstain from “outing” themselves even when speaking anonymously to pollsters. The full study is awaiting publication in Social Psychological and Personality Science journal but a pre-print version is available here.

Even more people say that their definition of “god” is simply a unifying force between all people. Or that they aren’t sure what they believe. If you lack an active belief in gods, you are an atheist. 

Being an atheist doesn’t mean you’re sure about every theological question, have answers to the way the world was created, or how evolution works. It just means that the assertion that gods exist has left you unconvinced.
Wishing that there was an afterlife, or a creator god, or a specific god doesn’t mean you’re not an atheist. Being an atheist is about what you believe and don’t believe, not about what you wish to be true or would find comforting.
All atheists are different

The only common thread that ties all atheists together is a lack of belief in gods. Some of the best debates we have ever had have been with fellow atheists. This is because atheists do not have a common belief system, sacred scripture or atheist Pope. This means atheists often disagree on many issues and ideas. Atheists come in a variety of shapes, colors, beliefs, convictions, and backgrounds. We are as unique as our fingerprints.

Atheists exist across the political spectrum. We are members of every race. We are members of the LGBTQ* community. There are atheists in urban, suburban, and rural communities and in every state of the nation."