613 Mitzvot
Negative Mitzvah 33
Not to be superstitious

Devarim (Dueteronomy) 18:10
Lo-yimatse vecha, ma'avir beno-uvito ba'esh, kosem ksamim, me'onen umenachesh umchashef.
There should not be found among you, who makes his son or daughter pass through the fire, or divine divinations, or observes times, or observes signs and omens or practices witchcraft.

The practice mentioned here is nachash in Hebrew. It means to foretell the future through means of personal experiences. Rambam says that this is the art of attempting to determine the future by association with current or common events. It is a form of soothsaying. Rashi said concerning observing omens, "His bread fell from his mouth, a deer blocked him on the road, his staff fell from his hand... such are the things he uses to decide his future."

It is basically superstitious or "luck." Saadia taugth that it is forbidden to be concerned with supersitions or bad omens. Neither shall we act upon them. (Chinukh 249) For example, if see a black cat, and think that something bad will happen, then we have disobeyed this mitzvah.

Bereshit (Genesis) 30.27
Lavan said to him, "If now I have found favor in your eyes, stay here, for I have divined (nachash) that YHVH has blessed me for your sake."

This is just one of the idolatrous practices that Lavan was involved with.

Yeshayahu (Isaiah) 8:19,20
When they tell you, "Consult with those who have familiar spirits and with the wizards, who chirp and who mutter:" shouldn’t a people consult with their Elohim? Should they consult the dead on behalf of the living? Turn to the Torah and to the testimony! If they don’t speak according to this word, surely there is no morning for them.

Instead of turning to superstition to determine our actions, we should turn to the Torah.

Tehillim (Psalms) 119:105
Your word is a lamp to my feet, and a light for my path.