Titans were the giant gods who were the offspring of Uranus and Gaia. Hesiod lists twelve of them in his Theogony, namely Oceanus, Coeüs, Crius, Hyperion, Iapetus, Theia, Rhea, Themis, Mnemosyne, Phoebe, Tethys and Cronos. These names are a mixed bunch of Greek and non-Greek origins, some expressing abstract concepts (such as Themis and Mnemosyne), others being more specific and 'concrete'.
The collective name Titans was said by Hesiod to derive from titaino, 'to stretch', 'strain' with a blend of tisis, 'vengeance' (see Tisiphone), this referring either to their procreative efforts or being an allusion to their relations with their father (Cronos overthrew Uranus for his high-handed behaviour).
Needless to say, there have been many attempts since Hesiod to explain the name, ranging from Jane Harrison's titanos, 'white earth', 'chalk' (so that they were originally men who painted themselves white to perform initiation rites) to Thomas Keightley's reduplicated taia, itself an alleged variant of aia or gaia, 'earth' (so that they were 'earth men').
Current thinking proposes, albeit tentatively, that the name may derive from Ho, 'to esteem', 'honour', 'respect', so that they are 'the honoured ones'. Their name lives on in the English 'titanic' to mean 'gigantic', 'colossal'.