What is January Barometer?

Legal Definition
The January barometer is the hypothesis that stock market performance in January (particularly in the U.S.) predicts its performance for the rest of the year. So if the stock market rises in January, it is likely to continue to rise by the end of December. The January barometer was first mentioned by Yale Hirsch in 1972.

Historically, if the S&P 500 goes up in January, the trend will follow for the rest of the year. Conversely if the S&P falls in January, then it will fall for the rest of the year. From 1950 till 1984 both positive and negative prediction had a certainty of about 70% and 90% respectively with 75% in total. After 1985 however, the negative predictive power had been reduced to 50%, or in other words, no predictive power at all.
-- Wikipedia