The words defective and deficient both share the same grammatical function; they’re both adjectives. However, these words have related but different meanings. Defective means faulty, but deficient means lacking something necessary.
Consider the example sentence in the cropped graphic above. Here the writer intends to communicate that the alloy has less of a constituent than expected. The alloy is therefore deficient (missing some ingredient) and not defective (faulty).
Don’t be defective in using deficient. Use the right word to employ precision of language in your writing. You’ll produce more effective presentations of your message. And that in turn will better represent both you and your brands in the minds of your audience.