There are a lot of definitions out there, but essentially it's the intersection between research evidence, clinical expertise, and patient preferences. Some more examples:

• Evidence-based nursing is an approach to health care practice that enables nurses to provide the highest quality care based on the best evidence available to meet he needs of their patients. - Melnyk & Fineout-Overholt, 2005
• The integration of the best research evidence with clinical expertise and patient values. - Sackett et al., 2000
• Synthesizing scientific evidence to improve quality and effectiveness in health care.  AHQR, 2012

• Assures patients receive the most up-to-date care possible
• Assists practitioners in dealing with massive volume of nursing and medical research in the literature
• Encourages patients and nurses to work together to make informed decisions
• Fosters professional responsibility

Looking at EBP from a broad, philosophical perspective, we can see three main stages:

The hardest part of EBP is often formulating a question -- you have to gather the pieces of puzzle before you can start putting it together. 

Enter the PICO(TT) model:

P - Population 

I - Intervention

C - Comparison or Control

O - Outcome (desired or of interest)

T - Type of Question (Is this a diagnosis, therapy, prognosis, etiology/harm, or prevention question?)

T -Type of Study Design (What study design would best answer this question? RCT, Cohort, Case Series, etc.

Question: In POPULATION, does INTERVENTION as compared to COMPARISON/CONTROL GROUP result in OUTCOME?

This mnemonic device helps us frame out question before we start searching for evidence. You may not have all the pieces depending on your type of question, but it's a great jumping off point.