The cis-trans definition is unambiguous only when you have two different groups on one of the alkene carbons and the same two groups on the other carbon, as in but-2-ene.
Then the two identical methyl groups are either cis or trans to each other, and the two identical hydrogen atoms are either cis or trans to each other.
The ambiguity comes from the definition of "similar groups".
Consider a molecule like 3-methylpent-2-ene
Some chemists argue that the structure above is trans-3-methylpent-2-ene because the identical methyl groups are on opposite sides of the double bond.
Other chemists argue that this is cis-3-methylpent-2-ene because the main chain passes through the double bond with its similar alkyl groups (methyl and ethyl) on the same side of the double bond.
Note that the cis and trans prefixes are always in italics and they are joined by a hyphen to the name of the molecule.
The Cahn-Ingold-Prelog convention removes this ambiguity.