Technically lossless, or real lossless, means that when the output is decoded by a conformant decoder, the result is mathematically identical to the input.
Visually lossless is a non-rigorous term that indicates subjective transparency i.e. output which to the lay human eye looks indistinguishable from the original. The output may look different if a viewer scrutinizes it in fine detail, but not in normative viewing conditions. As to how 'visual-losslessness' is determinined: there are video quality metrics, like PSNR and SSIM, that provide a quantitative measure of the difference of an image with respect to a reference. Subjective opinion tests have been carried out to establish threshold values for these metrics that can be used to define 'visual losslessness'
From Performance Analysis of Visually Lossless Image Compression,
Note that there are several visual quality metrics (indices)
such as MSSIM , PSNR-HVS-M , WSNR  and some others
that are able to characterize well enough quality of lossy
compressed images . Recently, it has been shown for
grayscale images  that it is enough to provide MSSIM
values larger than 0.985 or PSNR-HVS-M values larger than 40dB
in order to guarantee practically invisibility of distortions
in lossy compressed images.
As for re-encoding resilience, depends on what the goal of the re-encoding is. For viewing-only or casual re-editing purposes, CRF 18 in x264 should be fine. For professional re-editing, I would drop the CRF to, say, 12 or 14. Information, once lost, can't be perfectly recovered. But you can set your own standards by encoding at different CRFs and comparing manually. See https://video.stackexchange.com/a/17639