Samasapthama is when the charts are matched for marriage and each others rashis are opposite each other. For example if the rashi of the boy is Mesha and the of the girls is Tula, then it is called samasapthama. 1-7 from each other.
Samasapthama rashi's are considered very compatible, if other points are good.
The other word used likewise is Sashasthaka 6-8. This is used to study the placement of two planets or transiting from each other in mutual 6-8 position. This position is not good since it does not give good results to the native concerned.
For ex. If Saturn and Jupiter are in 6-8 from each other, the dasa of Saturn/Jupiter or Jupiter/Saturn will not be good to the native.