Side is a term that you'll see as a column header on the Unusual Whales flow feed. It is most often used to reference where along the Bid-Ask spread a trade took place. 

Every trade will have be tagged with a side. There are four possible sides:

Ask - A transaction that took place at or closer to the ask will have the side ASK. 

Bid - A transaction that took place at or closer to the bid will have the side BID.

Mid - A transaction that took place exactly between the bid and the ask will have the side MID.

None - The previous 3 labels made reference to where along the Bid-Ask spread a trade took place. The NONE tag is used in situations where the Bid-Ask spread may be irrelevant. Cross trades, trades that are out of sequence or late,  or trades which have been modified or cancelled will be tagged NONE.