In biochemistry, a ligase is an enzyme that can catalyze the joining (ligation) of two molecules by forming a new chemical bond.
For example, DNA ligase can join two complementary fragments of nucleic acid by forming phosphodiester bonds, and repair single stranded breaks that arise in double stranded DNA during replication.
In general, a ligase catalyzes the following dehydration reaction, thus joining molecules A and B:
[1] There are also some ligases that use the name "carboxylase" to indicate that the enzyme specifically catalyzes a carboxylation reaction.
To note: biochemical nomenclature has sometimes distinguished synthetases from synthases and sometimes treated the words as synonyms.