Theta
The expected change in an option’s price per day from the passage of time alone, quoted as a negative number for long options. A theta of −0.05 means the option sheds about $5 per contract per calm day.
Theta is the rent on optionality. It concentrates at the money and accelerates into expiration, which is why short-dated ATM options decay dramatically over a quiet weekend.
Reading theta in isolation misleads: a position collecting $50 a day in theta is also short the corresponding gamma. Practitioners think in the pair, never the single Greek.
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