Easein

Ease-in is a term used in animation to describe a timing function where an animated property progresses more slowly at the start and then speeds up toward the end. This creates a sense of natural acceleration, as if the motion is being influenced by a force that builds over time.

In web technologies, ease-in is commonly provided as a predefined timing function for transitions and animations. In CSS, transition-timing-function: ease-in corresponds to a cubic-bezier curve (0.42, 0, 1, 1). The same or equivalent curves can be used with the Web Animations API and in many JavaScript animation libraries, where ease-in is part of a family of easing options such as ease-out and ease-in-out.

Conceptually, ease-in is one of several easing presets intended to mimic physical movement. Others include linear (constant speed), ease-out (starts fast and ends slow), and ease-in-out (slow at the start and end with faster motion in the middle). Some libraries also provide more specific ease-in variants, such as easeInQuad or easeInCubic, which apply polynomial curves to shape the acceleration.

Practical use of ease-in appears in user interface design and motion graphics to convey subtlety and realism. It is often chosen for micro-interactions like button presses, accordions, or transitions where a gentle start improves perceived smoothness and readability. Designers may combine ease-in with complementary curves (e.g., ease-in-out) to control motion across multiple elements consistently.

Overall, ease-in is a foundational easing option that governs how an animation ramps up, contributing to the perceived naturalness of motion in digital experiences.