Halliday & Matthiessen (1999: 214, 215):
In transforming experience of time into meaning, human communities have evolved a number of basic parameters. We can identify four of these that are relevant in the present context:(1) the temporal staging of a process: it may be beginning, taking place or ending.(underlying concept: a process occupies a certain measure of time)(2) the temporal perspective on a process: we may frame it in or out of temporal focus. This takes many different guises in different languages, and even within the same language; such as(a) in focus: ongoing, out of focus: terminated;(b) in focus: significant in itself, out of focus: significant for what follows;(c) in focus: actualised, out of focus: visualised.(It is the last of these that is relevant to English.)(underlying concept: a process relates to the flow of experience as a whole, including other processes)(3) the temporal profile of a process: it is either unbounded or bounded.(underlying concept: a process has the potential for being extended in time)(4) the temporal location of a process: it can be related to 'now' as past, present or future.(underlying concept: a process takes place within a linear flow or current of time) …
When these parameters are grammaticised, they are referred to respectively as
(1) phase,(2) aspect,(3) aktionsart,(4) tense.