A symmetrical alkene is an alkene in whose molecule the doubly bonded carbons bear the same ligands.

eg. 1:

In 1, each double bonded carbon bears two methyl groups.  Thus, 1 is a symmetrical alkene.

eg. 2:

In 2, each doubly bonded carbon bears a methyl group and a hydrogen atom.  Thus, 2 is a symmetrical alkene.

eg. 3:

In 3, each doubly bonded carbon bears a methyl group and a hydrogen atom.  Thus, 3 is asymmetrical alkene.

In an organic molecule a carbon-carbon double bond in which the carbon atoms bear the same ligands is called a symmetrical double bond.

eg:

see also unsymmetrical alkene