An ideal filter (system) is the one which can be described mathematically but cannot be implemented (realized) physically.
Coming to a sinc filter, the ideality of this filter results from its frequency domain definition which is an ideal low-pass filter with zero ripple in the pass and stop bands and zero transition width.
Having these specifications, an ideal lowpass filter, can be described mathematically but cannot be realized using any techniques. However you can still find the resulting impulse response of an ideal lowpass filter by using the formal frequency to time transformation of the frequency reponse of the ideal filter.
This kept in mind, therefore, an ideal low-pass filter is seen to have an impulse response in the form of a sinc(x) signal, which extends from minus to plus infinity in time, which is another manifestation of the ideality of the filter being designed.
When such a filter is to be implemented in practice, it can only be approximated. The approximation being better as the length of the filter increases which however is a very undesired condition.
Instead of forcing approximations to ideal sinc filters this way, it is practically more effective to use other techniques such as weighted windowing to realize those filters, which results in the conversion of filter characteristics from those of the ideal one to the realizable one.