Is it possible to use a Canvas app inside a Lightning app (using a component)? Because to use canvas, we need to use
iframe, which can not be used in lightning.
Actually that is not correct - with Winter '16 we not only fixed the issues with Canvas and Lightning Experience/S1 (the way canvas was originally integrated into these applications actually violated the Lightning security model and its functionality was blocked as it was supposed to be). This has been corrected and in addition we have released the
force:canvaApp component as a new standard component.
Take a look at the doc at this link when logged into a Winter '16 org:
Btw you can also definitely use iframes with LC (a number of my solutions on stackechange use iframes to solve various problems) - not sure where that misinformation is coming from.