This is the belief that it is possible to make it into heaven without accepting Jesus Christ as savior.
Inclusivism takes numerous forms. Some claim that those who are unevangelized or those with very limited knowledge of the gospel will either be excused or get a second chance to accept Christ as savior. This is typically what is thought of as inclusivism. It is also known by some as “limited universalism” although the phrase itself would appear to be a contradiction of terms.
Universalism claims that all will make it to heaven because of God’s love. Inclusivism works under the same reasoning but with a more limited scope.
There are many who believe in an age of accountability or that those who are mentally handicapped will not be held accountable for their sins in the same way as the rest of humanity. This is typically not considered to be inclusivism however.
For further reading on different interpretations of what happens at death, see the article Fate of the Unbeliever.