According to Mendel something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’ now called as genes. Genes, therefore, are the units of inheritance.
They contain the information that is required to express a particular trait in an organism.
Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene.
There is no ambiguity that the genes are located on the DNA, it is difficult to literally define a gene in terms of DNA sequence.

The DNA sequence coding for tRNA or rRNA molecule also defines a gene. A cistron is defined as a segment of DNA coding for a polypeptide, the structural gene in a transcription unit could be said as monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria or prokaryotes).