Delta vs Theta

The rate of change of an option's price relative to a $1 change in the underlying asset price; ranges from 0 to +1 for calls and -1 to 0 for puts; ATM options have delta approximately ±0.5; delta is also used as a proxy for the probability that the option expires in-the-money.

The rate of change of an option's price relative to the passage of one day, all else equal; always negative for bought options (time decay reduces value); always positive for sold options; accelerates as expiry approaches; measured in dollars lost per day from time decay.