A slightly more complicated definition would be one who believes in the divinity of Jesus Christ and strives to live their life in accordance with His teachings.
Of course, depending on who you ask, this means different things.
As you probably already know, Christians are not one collective bunch.
Like in most religions, disagreements over theology and dogma have resulted in everything from quiet splits to devastating wars, in the past and even today.
rather than some specific moral or practical question.
In fact, you can make a good case that a defining attribute of Christianity is the wide variety of ways the teachings are applied in religions.
Catholicism is generally best known for its rituals and a rather authoritarian approach to religious and moral doctrine.
It is believed that the church's teachings on these subjects are "infallible" — without error — because the Holy Spirit will not allow the Church to be in error; debate remains, however on how to interpret this infallibility.
Note that some liberal denominations reject the notion of external salvation entirely, and only focus on Jesus' message of compassion and forgiveness while not focusing so much, if at all, on His teachings on personal morality and sanctification.
Others keep the focus on the personal morality and sanctification, but express it through acts of charity and giving — many major charity organizations around the world are run by Christians, and many hospitals and ambulances worldwide originated as Christian organizations.