How can I describe someone who feels little or no emotion?

The medical term is blunted affect. A more extreme case is called a flat affect.

From Wikipedia:

Blunted affect is the scientific term describing a lack of emotional reactivity on the part of an individual. It is manifest as a failure to express feelings either verbally or non-verbally, even when talking about issues that would normally be expected to engage the emotions. Expressive gestures are rare and there is little animation in facial expression or in vocal inflection

Blunted affect may be the noun, but if you're looking for an adjective to describe someone like that the term is affectless.

affectless : showing or expressing no emotion; also : unfeeling

If you're looking for a term to use in everyday conversation (your request for an appropriate medical term aside), it's common to describe someone as being emotionally detached.

Possible synonyms:

• reserved / suppressed
• restrained / self-restrained / self-contained
• discreet
• overinhibited
• dazed