The power of seeing things invisible to others; the power of foreseeing future events by means of shadows thrown before them. Many Highlanders claim this power, which the ancient Gaels called shadow-sight (taischitaraugh).
Nor less a vailed his optic sleight, And Scottish gift of second sight.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Second Sight from Fact Monster: