A Doubting Thomas is someone who will refuse to believe something without direct, physical, personal evidence; a skeptic.
The term is based on the Biblical

Bible

The Bible refers to any one of the collections of the primary religious texts of Judaism and Christianity. There is no common version of the Bible, as the individual books , their contents and their order vary among denominations...

 account of Thomas

Thomas the Apostle

Thomas the Apostle, also called Doubting Thomas or Didymus was one of the Twelve Apostles of Jesus. He is best known for questioning Jesus' resurrection when first told of it, then proclaiming "My Lord and my God" on seeing Jesus in . He was perhaps the only Apostle who went outside the Roman...

 the Apostle, a disciple of Jesus

Jesus

Jesus of Nazareth , commonly referred to as Jesus Christ or simply as Jesus or Christ, is the central figure of Christianity...

 who doubted Jesus' resurrection

Resurrection

Resurrection refers to the literal coming back to life of the biologically dead. It is used both with respect to particular individuals or the belief in a General Resurrection of the dead at the end of the world. The General Resurrection is featured prominently in Jewish, Christian, and Muslim...

 and demanded to feel Jesus' wounds before being convinced (John 20:24-29). After seeing Jesus alive and being offered the opportunity to touch his wounds, according to the author of the Gospel of John

Gospel of John

The Gospel According to John , commonly referred to as the Gospel of John or simply John, and often referred to in New Testament scholarship as the Fourth Gospel, is an account of the public ministry of Jesus...

, Thomas then professed his faith in Jesus.