špekulacija

1. (guess, rumour) speculation; conjecture
   To su samo špekulacije, niko ništa ne zna sigurno. — That's just speculation, nobody knows anything for sure.
   Mediji su puni špekulacija o njegovoj ostavci. — The media are full of speculation about his resignation.
2. (in finance) speculation
   Cene stanova rastu zbog špekulacija investitora. — Flat prices are rising because of investor speculation.

In the sense of rumours or guesses it is mostly used in the plural.