Absolute tense is a temporal category that locates a situation (or a topic time) to an extra-linguistic reference point, typically the moment of utterance.
Absolute tenses typically refer to the present moment (the time of utterance) as the deictic centre. Absolute tenses thus locate the time of the situation or the topic time relative to the time of speech/time of utterance.
Three absolute tenses are commonly distinguished: (i) present tense, past tense and future tense. Present tenses locate situations at or around the time of utterance. Past tenses locate situations prior to the time of utterance, and future tenses indicate that a situations will take place after the time of utterance.
Rather than exhibiting a three-way distinction like the one pointed out above, many languages have a two-way split, e.g. distinguishing between past and non-past or between future and non-future.
Comrie, B. (1985). Tense. Cambridge: Cambridge University Press.