A thial, or thioaldehyde, is any molecule with the group RC(S)H. It is like an aldehyde with the oxygen atom changed with a sulfur. The sulfur has a double bond to carbon. It also keeps two lone pairs.
Thials are very reactive because they are very good electrophiles. It is difficult to make one, because they will react with themselves as soon as they are formed. They can do Diels-Alder reactions very quickly. However, if bulky R groups are used, the thial might be more stable.
If the H is changed with another R group, then the molecule is called a thioketone. These are more stable.