Tense is a form of a verb which indicates the speaker’s perspective or viewpoint of the time of action and the kind of action of the verb.
The time of action can be past, present or future. The kind of action can be mere occurrence, duration or completion.
The Indicative tense in Greek has 10 forms:
present tense generally refers to present time and durative action
imperfect tense generally refers to past time and durative action
aorist tense generally refers to past time and mere occurrence
2nd aorist is different in form but has the same meaning as aorist
perfect tense generally refers to past time and completed action
2nd perfect is different in form but has the same meaning as perfect
pluperfect tense generally refers to (more remote) past time and completed action
2nd pluperfect is different in form but has the same meaning as pluperfect
future tense refers to future time and any of the kinds of action
2nd future is different in from but has the same meaning as future
Older Greek grammars often refer to tense while some newer grammars call this form in order to emphasis verbal aspect. A modern approach is to refer to the form (or morphological encoding) as tense form, while referring to the concept of the viewpoint of the action as aspect.