Unconditionally secure cipher:

An encryption scheme is said to be unconditionally secure, when there is not enough information in a ciphertext to determine uniquely the plaintext, regardless of how much ciphertext available. i.e., provably immune to the compute power or any form of attack.

Unfortunately there is no encryption algorithm that is unconditionally secure, with the exception of the one-time pad(OTP)

Computationally secure cipher:

The encryption technique is computationally secure if

the cost of breaking the cipher exceeds the value of the encrypted information or

the time required to break the cipher exceeds the useful lifetime of the information.
even if it met only one of these, it is said to be computationally secure.

but, The amount of time needed to properly cryptanalyze ciphertext is difficult to estimate.