“Alt Season” is the part of the overarching cryptocurrency market cycle where many altcoins “moon” (go up quickly in price) against the dollar and Bitcoin at once (for example, late December 2017 – early January 2018).
In general altcoins tend to be in a perpetual state of bearishness or stagnation, either hovering in price or going down. Then, once in a while, an alt or two will catch an uptrend or epic run (generally a pump). However, every so often [like once or twice a year if we are lucky] all alts will “moon” at once or in quick succession for an extended period (for example a week, a few weeks, or even a month or two).
This period in which all altcoins are running together is sometimes called alt season by cryptocurrency traders / investors on social media.
When alt season occurs altcoins tend to out preform Bitcoin and the Dollar. If you are looking for early signs of alt season, look to the price action of the top alts Ripple and Ether (subject to change).
TIP: Outside of “alt season” most alts can loose amazing amounts of value (either because Bitcoin goes up or because everything is in a bear cycle). So while going “long” in alts can make sense, you typically want to avoid going all-in at the height of alt season and holding. 1,000% gains are real, but so are 90% losses. The alt market is not for the feint of heart.