not a lottery. in case of a lottery, when you have the winning numbers you go to a central authority that manages the whole thing and you claim the winnings. Also, in case of a lottery the winnings are a share of what has been put into buying the tickets. also, lotteries can go a long time without having a winner for the jackpot. Also for lotteries, usually there is a tiered winning structure and what you win on is fixed (ie the numbers)

for bitcoin there is no central authority. the reward for winning is “fixed” and claiming the said reward is dependent on a network that validates that you indeed have the work that incorporates pending transactions into the blockchain. also, “winning” happens at regular intervals of time (average 10 minutes based on difficulty), and what you’re winning is somewhat fixed (the block reward is fixed + some TX fees var).

so it’s not a lottery.