The term coevalness refers to sharing the same timeframe. Much like 'medieval' refers to the middle ages, co-eval refers to a shared age.
Coevalness has become an important term in the critique of colonialism, structuralism and the like since Johannes Fabian's Time and the Other (1983). In this book, Fabian demonstrated how, in anthropological writing, the Other was often described in terms that set the Other in an earlier age than ours (i.e. that of the western anthropologist), by means of small phrases such as 'people still live according to the rhythm of the seasons', they 'practice ancient rituals', and live their lives 'as if time stood still'. This, Fabian argued, was a feature of evolutionism, in which we - the sophisticated West - represented modernity while the exotic Other represented pre-modernity. He called this mechanism the denial of coevalness: we deny others their presence in the same age as ours.
The denial of coevalness can be found in travel writing too, and it is (implicitly) central to Edward Said's famous argument in Orientalism (1978): the Orient, even if observed in the present, was consistently pictured as backward, reflecting an earlier age of lower development. Conversely, in science fiction the Other would typically be described as ahead of us, representing an even more advanced level of civilization than ours.