Articles may be divided into two types: definite (the) and indefinite (a, an). Articles are a type of determiner and indicate the specificity or nonspecificity of the noun or noun phrase they modify.
a specific airflow velocity
an electronic circuit
the Greenhouse Effect
Pay close attention to usage rules governing whether an article is necessary with a particular noun or noun phrase and, if so, whether the required article should be definite or indefinite.