Dictionary Definition:

“Having an oxygen atom linking two alkyl or other organic groups.”

Selected from OxfordDictionaries.com

Functional Definition: 

Ethers are functional groups that consist of the R-O-R structure in which the R groups are the organic carbon containing elements of the molecule. The oxygen has two single bonds to the carbon elements with two electron pairs.

Reactions with ethers require a strong acid like HCl or HBr and complete with a nucleophilic substitution. Essentially two molecules of Cl- or Br- or other halogen take the place of oxygen and break the molecule in half. Half is where the oxygen held the two sides together, producing water and two R-Cl or R-Br for example.

Ethers are common in pesticides such as glycol ether and  ethyl ether and herbicides like the no longer used Agent Orange.