Allylic strain, or A strain is closely associated to syn-pentane strain.  An example of allylic strain can be seen in the compound 2-pentene.  It's possible for the ethyl substituent of the olefin to rotate such that the terminal methyl group is brought near to the vicinal methyl group of the olefin.  These types of compounds usually take a more linear conformation to avoid the steric strain between the substituents.