HODL - misspelling of hold and it means keep; hold that coin and don't sell it
FOMO - fear of missing out; a recent example is Bitcoin Gold fork, because people started to buy bitcoin like crazy even if the price was very high at some points
FUD - fear, uncertainty and doubt; some spread FUD to crash the coin price
ATH - all time high - the highest historical price of specific coin
Whale - a person who has a lot of coins ( a substantial amount of capital ); he/she can influence the price by selling
Bag holder - a person who bought at around ATH and never sold and now he/she has a lot of worthless coins
Long - long position => assuming the value of the coin will be higher in the future
Short - short position => assuming the value of the coin will be smaller in the future
BUY/SELL Wall - limit orders of the same price target from a specific exchange
Altcoin - alternate coin; everything other than BTC
ICO - initial coin offering