Any text on your website, that carries the same color as its background (or anything closer!), or carries too low a font size that makes it barely visible to human eyes, qualify to be called as ‘invisible’ text.
Invisible Text has been used for keyword spamming and ever since search engines detected it, this method has been treated to be unethical SEO. Every Search Engine has now refined its algorithm to detect spam text and they blacklist any websites found to be following such unethical practices. Hence Invisible Texting is rarely practiced by any websites these days.