Isomer Identification Using Condensed Structural Formulae
isomer
One of several species (or molecular entities ) that have the same atomic composition (molecular formula) but different line formulae or different stereochemical formulae and hence different physical and/or chemical properties.

Source:
Pure and Applied Chemistry, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
Pure and Applied Chemistry, 1996, 68, 2193 (Basic terminology of stereochemistry (IUPAC Recommendations 1996))
a)
C5H10
and
C10H20
different chemical formula → not isomeric
b)
CH3(CH2)4CH3 = hexane (C6H14)
and
CH3(CH2)3CH3 = pentane (C5H12)
different chemical formula → not isomeric
c)
CH3CH(CH3)(CH2)2CH3 = 2-methylpentane (C6H14)
and
CH3(CH2)2CH(CH3)2 = 2-methylpentane (C6H14)
same chemical formula and same structural formula → identical, not isomeric
d)
(CH3)3CH = isobutane (C4H10)
and
CH3CH2CH2CH3 = butane (C4H10)
same chemical formula and different structural formula → isomeric