The term "isomer" in organic chemistry, refers to two or more substances that have the same molecular formula but different structures and somewhat different physical and/or chemical properties.
These differences in structure could be due to how the carbon atoms are connected in the compounds (structural isomers), or due to how the atoms around a double or triple bond are connected to the carbon atoms (stereoisomers).
An example of an isomer would be butane which has the chemical formula of C4H10. The four carbon atoms could be connected in a straight line to form a linear molecule, or there could be three carbon atoms connected in a line, with the fourth attached to the middle carbon of the first three. These would be structural isomers.
The examples you list all have different molecular formulas:
pentane = C5H12
pentene = C5H10
dichloropentane = C5H10Cl2
bromochlorfluoromethane = CHBrClF
so none is an isomer of any of the others.
Now, pentane can have several different structural isomers such as 2-methyl butane and 2,2 - dimethylpropane.
Likewise, pentene - which contains one double bond - can have multiple structural isomers (at least 5) as well as multiple stereoisomers because of the double bond present in this compound