Today, February 29th, is a leap day, defined by the OED as an intercalary day. This is a day inserted into the calendar to synchronise the calendar year with the solar year and with the seasons. So, why leap? Where does the idea of leaping come from? In non-leap years, fixed-date festivals fall one day of the week later than in the previous year, so if Christmas Day is on a Monday one year, it falls on a Tuesday the next year. In a leap year the fixed-date festivals from March onwards fall two days of the week later than in the previous year, so Christmas Day in 2015 was on a Friday, whereas this year it will be on a Sunday i.e. it will leap forward by an extra day. The OED thinks that this is why we use the term leap. Or, it may be because medieval Latin and Old English referred to a leap of the moon (saltus lunae and mónan hlýp respectively). This, in the words of the OED, was "the omission of a day in the reckoning of the lunar month, made every nineteen years to bring the calendar into accord with the astronomical phenomena".
Another term for a leap year is bissextile, from the Latin bis, twice and sextus, sixth. And why sixth? Because the intercalary day was "inserted by the Julian calendar every fourth year after the sixth day before the calends of March, or 24th of February" (OED). The calends, or kalends, of March was 1st March.
The OED also includes the obsolete idiom 'to make leap year of', meaning 'to pass over'.
There's lots more information on leap years in this Daily Telegraph article.