The aorist is the simple past tense.

The term "aorist" comes from the Greek, meaning "undefined" or "not specified." It is the tense used when one wishes to express the type (aspect) of action as mere occurrence, without reference to completion (perfect) or to duration or repetition or attempt (imperfect).

That does not mean that the aorist is the opposite of the perfect or the imperfect; it simply means that those matters are not specified. More often than not, it will be the case that simple action is meant, and grammarians usually speak of this as "point" action--action that occurs in an instant, and is over.

Note the differences in the following examples.

• He was opening the door. imperfect
  (The act itself is emphasized in various ways.)
• He has opened the door. perfect
  (done, and the door remains opened)
• He opened the door. aorist
  (done, but nothing more implied)

The simple past tense in English is usually formed by adding "ed" to the verb.

The Greek aorist tense covers a wider range than the English simple past tense, often including what would be expressed in the perfect tense in English. On the other hand, Greek has a more narrow perfect tense than does English.

Of the seven Greek tenses, only the present and aorist are used regularly outside of the indicative mood. In other moods than the indicative, aorist indicates aspect, not tense. It indicates point action rather than continuing action, which is represented by the present.