【Keep】 ,  【keep back】 ,  【keep out】 ,  【retain】 ,  【detain】 ,  【withhold】 ,  【reserve】 ,  【hold】 ,  【hold back】  are comparable in meaning not to let go from one's possession, custody, or control.

【Keep】 is the most general of these terms, often carrying no further implications. When, however, it positively denotes a holding securely in one's possession, custody, or control,  【keep】 , or more often  【keep back】 , is synonymous with one or another of the remaining terms.

【Keep out】  specifically implies a keeping back of a portion of something <kept out a part of his salary for emergency expenses>

【Retain】 implies continued keeping, especially as against threatened seizure or forced loss.

【Detain】  (see also ARREST 2 ) implies a keeping (as in a place, in conversation, or in one's possession or control) through a delay in letting go that may be based on selfishness or caprice or on entirely acceptable grounds.

【Withhold】  implies restraint in letting go or refusal to let go. Sometimes it is interchangeable with  【keep】 , or  【keep back】 , especially when hindrance is also implied.

Sometimes,  【keep】  and  【withhold】  are widely different in meaning; thus, to  【withhold】  one's promise is to refuse to give one's promise; to  【keep】  (see  【KEEP】 1 ) one' promise is to fulfill what has been promised.

【Reserve】 implies either a keeping in store for other or for future use or a withholding from present or from others' use or enjoyment.

【Hold】  and  【hold back】  are often used in place of  【withhold】  or  【keep back】  and sometimes in place of  【detain】  and  【reserve】 when restraint in letting go, whether self-imposed or imposed by others, is implied.