An ELT Glossary : Language transfer

Language transfer is the effect, which may be positive or negative, of the L1 on the learner's use of the target language. 

For example :  An Italian learner who has never heard the word "invention" before may still understand it because of its similarity to the Italian "invenzione"  - this is a positive effect; however, the same learner may be inaccurate in the use of articles when describing generic facts - eg saying *I like the horses because the definite article would be used in this way in Italian. This is therefore a negative effect.

The term was coined by Pit Corder in the 70s to react against the Audiolingual view of L1 influence as "interference" - ie inevitably negative and detrimental to learning.